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Brythonic Personal Names

The list below includes names taken from early Brythonic sources and those still used in Wales, Brittany and Cornwall today.

All forms have been standardised according to modern Welsh orthography, except modern Breton and Cornish forms, which are coloured blue and green respectively. Old Breton and Cornish forms with no modern equivalent have been given in standard modern Welsh and are marked with an asterisk (*).

Early sources used:

bgg Bonedd Gwŷr y Gogledd, MS 13th C., possibly from 12th C. source
bm Bodmin Manumissions, 9th - 10th C.
hg Harleian Genealogies, MS c12th C., possibly from 10th C. material
t Welsh Triads, late 13th C.
yg Y Gododdin, late 13th C. possibly from much earlier material.

Aeddan MW. Ædan, Aedan, OC. Aedan, E. Aidan. A borrowing from OI. Aedán (I. Aodhán), meaning 'little fire'. hg, bm, bgg, yg
Angharad (f.) Br. *Ancaratā 'well loved' (W. caraf 'I love').
Anarawd MW. Anarant (sic.). Probably from L. Honōrātus meaning 'distinguished'. hg
Anaostl* (f.) OC. Anaguistl.  Probably 'wealth hostage' with W. anaw 'wealth; wealthy' and gwystl 'hostage'. bm
Aneirin MW. Aneurin, Aneirin. Of uncertain origin. Perhaps from L. Nigrīnus < niger 'black', or Br. *Nagrīnos, a derivative of PC. *nagro- (OI. nár 'noble, honourable; modest', náire 'shame, bashfulness, modesty, decorum'). yg
Anthun E. Anthony. From L. Antōnius. hg
Arc'hantael (f.). B. arc'hant 'silver' and ael 'eyebrow'.
Artan, Arthan MW. Artan. Probably a borrowing from OI. Artán 'little bear', but it may be the Welsh cognate *Arthan. hg
Arthek Br. *Artācos 'bear-like' (W. arth 'bear').
Arthfoddw MW. Artbodgu. Br. *Artobodwos 'bear crow' (W. arth 'bear', OI. bodb, badb 'hooded crow'). hg
Arthial MW. Arthgal, I. Árdal. From PC. *Artogalos 'bear power' (W. arth 'bear', gallaf 'I am able'). hg
Arthien MW. Arthgen.  From Br. *Artogenos 'bear-born' (W. arth 'bear').  hg
Arthur B. Arzhur. Probably the most famous Brythonic name, but of uncertain origin. Probably from L. Artōrius. hg, t, YG
Asa, Asaph MW. Assa. Probably the Biblical names Asa or Asaph. bs
Awen (f.) The common noun W. awen 'inspiration, muse', perhaps related to awel 'breeze'.
Baglan MW. Baglan. W. baglan 'crook, crutch' from bagl 'crosier, crook'. bs
Bedwyr E. Bedivere. Probably Br. *Betwowiros 'birch man' (W. bedwen 'birch tree', gŵr 'man'). t
Beli MW. Beli, OC. Beli. Of uncertain origin, though a relatively common name in the king lists. Perhaps from Br. *Bolgjos, *Belgjos < PC. bolg- 'belly' (W. bola), or a name relating to the god Belenos. hg, bm, yg, t, bs
Berwyn MW. Berwin. Br. *Barrowindos 'white top' (W. bar 'summit, top, crest', gwyn 'white'). jc
Bizouarn OB. Budhoiarn. From B. buz 'profit' (previously 'victory') and houarn 'iron'.
Bleiddfan MW. Bleidvan.  Br. *Bledjomandus 'wolf pony' (W. blaidd 'wolf'). yg
Bleiddgwn* OC. Bleidcum (sic.). Br. *Bledjocunam (acc.) 'wolf hound' (C. bleydh 'wolf', ki 'dog'). bm
Bleiddig From W. blaidd 'wolf' and the diminutive -ig. yg
Bleiddudd MW. Bleydiud, Bleiddut, OC. Bleidiud. From Br. *Bledjojüd- 'wolf lord' (W. blaidd 'wolf', udd 'lord'). hg, bm, bs
Bodgad MW. Botgat. Br. *Bodwocatus 'battle crow' (OI. bodb, badb 'hooded crow', W. cad 'battle').yg
Boddw MW. Bodgu. Br. *bodwo- 'crow' (OI. bodb, badb 'hooded crow'). hg, yg
Bradwen MW. Bratwen. Seemingly a compound of W. brad 'treachery, treason' and gwen 'white'. yg
Brân MW. Bran. Br. *Branos 'crow, raven' (W. brân 'crow', OI. bran 'raven'), a bird commonly associated with battles. hg, t
Branek From Br. *Branācos 'raven-like' (C. bran 'crow')
Breichiol MW. Brechiaul. Possibly L. Bracchiālis < bracchium 'arm' (W. braich 'arm, branch'), perhaps inferring 'having strong arms'. There is a Welsh word breichiol 'linked together; brachial'. hg, yg
Brengi* OC. Brenci. Probably Br. *Branocī 'raven hound' (C. bran 'crow', ki 'dog'). bm
Briog OC. Frioc, C. Bryok, B. Brieg. Br. *Brīgācos 'powerful, esteemed' (C. bri 'distinction, esteem, importance'). bm
Brochfael MW. Brochmail, Brocmayl. Br. *Broccomaglos 'badger prince' (W. broch 'badger', mael 'prince, lord'). hg
Bronwen (f.) Br. *Brondowindā 'white breast' (W. bron 'breast', gwen 'white').
Brezelgar* OB. Preselgar. The first element is B. brezel 'war' and the second may be B. kar 'friend'.
Brezelgevgant* OB. Breselcoucant. B. brezel 'war' and the equivalent of W. ceugant 'certain; special'.
Brezelgonan* OB. Breselconan. B. brezel 'war' and the personal name Br. *Cunagnos (see Cynan).
Brezelvarc'heg* OB. Breselmarcoc. 'War horseman' from a compound of B. brezel 'war' and marc'heg 'knight'.
Brezelek* OB. Breseloc. 'Warlike' from Br. *Bristelācos (B. brezelek 'bellicose').
Brythael* OC. Brithael. Perhaps Br. *Britosagelos 'generous of judgment' (C. bres 'mind, thought', hel 'generous'). bm
Buan MW. Buan. Probably the common adjective W. buan 'swift, nimble'. bs
Buddfan MW. Budvan. Br. *Büdomandus 'victory pony' (W. budd 'profit, gain, riches'). yg
Buddug OC. Budic, OB. Budoc, B. Beuzec E. Boudica. From Br. *Büdīcos, Büdīcā 'victorious' (C. budhek 'victorious', W. buddig 'victorious, successful, generous'). The Breton forms are from equivalent Br. *Büdācos. A female name in Welsh, but apparently masculine in Old Cornish and Old Breton. bm
Buzaret OB. Buduuoret. B. buz 'profit' (previously 'victory') and gwared 'protection'.
Buzval OB. Butgual. The first element is B. buz 'profit' (previously 'victory') and the second may be Br. *walo- 'leader' (see Cadwal).
Cadan Probably a diminutive of names in Cad-.
Cadell MW. Catell, Catel, Kadell. Probably from L. catellus 'puppy' (see Cenau). hg, bs
Cadfan MW. Catman, Katuan. Br. *Catumandus 'battle pony'.  hg, t, bs
Cadfannan MW. Catvannan. A dimunitive of Cadfan. yg
Cadfawr MW. Catmor. Br. *Catumāros 'great battle' (W. cad 'battle', mawr 'great'). MW. Catmor may be for Br. *Catumori 'battle-sea' or 'sea battle' (W. môr 'sea'). hg
Cadlew MW. Catleu. The elements may be W. cad 'battle' and glew 'warrior, hero'; less probably llew 'lion'. hg, yg
Cadog OB. Cadoc, B. Kadeg. From Br. *Catācos 'battle-like' (W. cad 'battle'). hg
Cadogan MW. Catgocaun, W. Cadwgan. Properly *Cadogawn or *Cadogon from Br. *Catuwocānos 'battle glory' (W. cad 'battle', gogawn 'glory, honour; satisfaction').  hg
Cadwal MW. Catgual, G. Cathal. Br. *Catuwalos 'battle leader' (W. cad 'battle').  hg
Cadwaladr MW. Gatgualart (sic.), Katwalaudyr, OB. Catuualart (sic.). Br. *Catuwalatros 'battle-leader' (W. cad 'battle'). hg, t, bs
Cadwallon MW. Catgollaun, Catguallaun, Katwallaun, OB. Catuuallon. Br. *Catuwallānos 'battle leader' (W. cad 'battle').  hg, t, bs
Cadwr MW. Catgur, OB. Catuur.  Br. *Catuwiros 'warrior, battle hero' (W. cad 'battle', gŵr 'man'). hg
Cadwystl* (f.) OC. Catguistl.  Br. *Catugēstlā 'battle hostage' (C. kas 'battle', gostel 'hostage'). bm
Caeog MW. Kayawc. From W. cae 'hedge, fence, enclosure', perhaps meaning 'protector'. yg
Caradog MW. Caratauc, E. Caratacus, Caractacus. Br. *Caratācos 'loving' or 'beloved' (W. caraf 'I love'). The modern Welsh ought to be *Cardog. hg, t, yg
Carannog MW. Carannauc. Br. *Carantācos 'dear, beloved' (W. carannog). bs
Cateyrn MW. Cattegirn. Br. *Catutigernos 'battle king' (W. cad 'battle', teyrn 'king'). hg
Cawrdaf MW. Caurtam, Caurdaf. Br. *Cawrotamos 'most giant' (W. cawr 'giant'). hg, bgg, t
Cedig MW. Kedic. Br. *Catīcos 'battle-like' (W. cad 'battle'). bgg, bs
Ceidio MW. Keidyaw, OB. Cadiou. A hypocoristic of names in Cad-. bgg, yg, t
Cenau MW. Ceneu, Keneu. From the common noun W. cenau 'whelp, puppy' < Br. *canawū. hg, bgg, yg
Ceredig MW. Ceretic, Keredic. Identical with Caradog, but with a different ending: Br. *Caratīcos 'loving' or 'loved'. hg, yg, bs
Cian MW. Gian. Either borrowed from I. Cían meaning 'ancient, enduring' or a late (post-Brythonic) diminutive of W. ci 'dog'. yg
Cilydd MW. Kilyd. From the W. common noun cilydd 'fellow, companion'. yg
Clodri MW. Clotri. Br. *Clutorīgam (acc.) 'famous king' (W. clod 'praise, fame, renown; famous', rhi 'king'). hg
Clydno MW. Clydno. From Br. *Clutognāwjos 'knowing fame' (W. clod 'praise, fame, renown'). bgg, yg, bs
Clydog MW. Clitauc, Clytauc, C. Klesek. From Br. *Clutācos 'famous; famed one' (W. clod 'praise, fame, renown'). hg, jc
Clydwyn MW. Clytwin. Br. *Clutowindos 'blessed fame' (W. (W. clod 'praise, fame, renown', gwyn 'white, blessed'). jc
Clynog MW. Clinog, Clinoch. The origins of this name are uncertain. W. clynnog occurs as a syncopated form of celynnog '(place) abounding with holly' in the place name Clynnog Fawr (Caernarfonshire). It is possible the personal name is the same word with the sense 'holy-like', in the vein of other names derived from trees. However, the records of this name are rather early for such syncope to have taken place and we might expect double -nn- to appear in the Middle Welsh spelling. There are no other clear candidates for the first element of the name. It is possible that this is merely a scribal error: the name only occurs in the Harleian Genealogy and the equivalent people are called Clydno and Kedic in the Bonedd Gwŷr y Gogledd.  hg
Coel MW. Coil, Coyl, Gyl, Coel. From the common noun W. coel 'belief; omen; reputation' < PC. *kailā (OI. cél 'omen, portent'). hg, bgg
Corun MW. Corun. From L. corōna 'crown, wreath' (W. coron 'crown', corun 'crown (of head), tonsure'). bs
Cunedda MW. Cuneda, Kuneda. A well-known name, probably from Br. *Cunodagos 'good hound' (W. ci 'dog', da 'good'). The modern Welsh form ought to be *Cyndda, but it has retained a much older spelling due to it being passed down through written records. hg, bs
Cyfwlch MW. Chyvwlch. The same as the adjective W. cyfwlch 'complete, perfect, excellent'. yg
Cynan MW. Cinan, Chynan. Br. *Cunagnos 'little hound' (W. ci 'dog', -an diminutive suffix). hg, bgg, yg
Cyndeyrn MW. Kyndeyrn, E. Kentigern. Br. *Cunotigernos 'hound king' (W. ci 'dog', teyrn 'king'). This cannot be from Br. *Cintutigernos 'first king', as is often stated, since this would produce W. *Cynteyrn. t, bs
Cynfan MW. Chynvan. Br. *Cunomandus 'hound pony' (W. ci 'dog'). yg
Cynfarch MW. Cinmarc, Kynuarch. Br. Cunomarcos 'hound horse' (W. ci 'dog', march 'horse'). hg, bgg, t
Cynfelyn MW. Cinbelim, Cinbelin, Chynuelyn, Kynuelyn. Br. *Cunobelinos 'hound of the god Belenos'. hg, bgg, t, bs, yg
Cynfynog* OC. Conmonoc.  Br. *Cunomonācos 'noble hound' (C. ki 'dog', W. mynog 'noble'). bm
Cyngar MW. Cincar, Kyngar, C. Kengar. Probably Br. *Cunocaros 'hound love' (W. ci 'dog', caraf 'I love'). hg, bs
Cyngen MW. Cincen. Probably Br. *Cunocennos 'hound skin' (W. ci 'dog', cen 'skin'). hg
Cynin MW. Cinnin. Br. *Cunignos 'little hound' (W. ci 'dog'). hg
Cynlas MW. Cinglas. Br. *Cunoglassos 'grey hound' (W. ci 'dog', glas 'blue, green, grey'). hg
Cynri MW. Kynri. Br. *Cunorīgam (acc.) 'hound king' (W. ci 'dog', rhi 'king'). yg
Cynwal MW. Gynwal. Br. *Cunowalos 'hound leader'. yg
Cynwr* OC. Cingur. Br. *Cunowiros 'hound man' (C. ki 'dog', gour 'husband'). bm
Cynwyd MW. Cinuit, Kynnuyt. Identical in origin to the river name Kent from Br. *Cunētjū, perhaps meaning 'health-giving'. hg, bgg, t
Cynyr MW. Gynyr. From the nominative of the same name which gives Cynri, Br. *Cunorīx 'hound king' (W. ci 'dog', rhi 'king'). bs
Deigion MW. Decion. Probably from L. Deciānus < Decius. hg
Deiniol MW. Deinyoel, E. Daniel. From L. Daniēl. bs
Dewi MW. Dewi. From L. Davīd. bs
Dinogad, Dingad MW. Dinacat, Dingat, Dinogat. Br. *Dīnocatus 'battle fort' (W. din 'fort', cad 'battle'). The modern Welsh ought to be *Dingad. hg, jc, yg, bs
Dogfael MW. Docuael. bs
Dunod MW. Dunaut, Dunawd. L. Dōnātus. hg, bgg, t, bs
Dwywai (f.) MW. Dewei. Probably a derivative of Br. *dēwā 'goddess' (W. duw 'god'). bs, yg
Dyfnwal MW. Dumnagual, Dyuynwal, Dyvynwal, Dyfynwal, B. Dunvel? G. Dòmhnall, E. Donald, Dunmail. From Br. *Dumnowalos 'world leader' (W. dwfn 'deep', G. domhan 'universe'). hg, bgg, yg, bs
Dyfnwallon MW. Dumnguallaun.  Br. *Dumnowallānos 'world learer' (W. dwfn 'deep', G. domhan 'universe'). hg
Edern MW. Edyrn, Ætern, Edern. From L. Æternus 'eternal'. hg, bs
Ednyfed MW. Iutnimet, Idnyuet, Edynyuet. Br. *Jüdonemetos 'noble lord' or 'lord of the sacred place' (Gaul. nemeton 'sacred grove', OI. neimed 'sanctuary; sacredness, status'. Properly *Idnyfed in modern Welsh. hg, bgg, bs
Eigion MW. Eiciaun. Presumably from Br. *Oceānus < L. Ōceanus meaning 'ocean' (W. eigion 'ocean'). hg
Eiludd MW. Eliud, OC. Iliud. Br. *Elujüd- 'many lords' (MW. udd 'lord'). hg, bm
Einion MW. Eniaun, Enniaun, Einyaun. Probably from L. Anniānus, Aniānus. The name is usually explained as being from W. einion, eingion 'anvil', which is possible, but the origins of that word are uncertain and the MW. -aun suggests Br. *-ān- for the name whilst C. anwan 'anvil' suggests Br. *-an- for the common noun. hg, bs
Eithinyn MW. Eithinyn. From W. eithin 'gorse, furze, whin'. yg
Elen (f.) L. Helena < Gk.  Ελενη. hg
Eleuther MW. Eleuther.  L. Eleutherius from eleutheria ‘liberty’. hg
Elfodd, Elfoddw MW. Elbodgu. Br. *Elubodwos 'many crows' (Gaul. bodua-, OI. bodb, badb 'hooded crow'). hg
Elffin MW. Elfin, Elffin, Elphin, G. Ailpein, Pict. Alpin. A name of uncertain origin, though L. Alpīnus meaning 'Alpine' would fit the Welsh phonetics perfectly. hg, bgg, yg, bs
Eli MW. Eli. Probably the Biblical name. yg
Eneas MW. Eneas. Biblical. bs
Erthgi MW. Erthgi. From Br. *Artocī 'bear hound' (W. arth 'bear', ci 'dog'). yg
Eudaf MW. Eudaf.  Br. *Awitamos 'most friendly'. yg
Fferfarch MW. Feruarch. Br. *Feromarcos, meaning either 'wild horse' or 'valiant horse' (see below). yg
Fferog MW. Ferawc. Perhaps Br. *Ferācos 'wild one' from L. ferus 'wild, fierce' (W. lledffer 'half-wild'), or from W. ffêr 'strong, valiant, ferocious; hero' < L. ferōx 'warlike'. yg
Garfonion MW. Garbaniaun, Garmonyawn. Probably L. Germāniānus, a derivative of *Germanus. hg, bgg
Geraint MW. Gereint.  From L. Gerontius. yg, t
Gloywfedd* (f.) OC. Gloiumed. Seemingly 'sparkling mead' (W. gloyw 'bright, shining, sparkling', W. medd, C. medh 'mead'). bm
Gloywgen* (f.) OC. Gloiucen. Probably meaning 'shining skin' (W. gloyw 'bright, shining, sparkling', C. kenn 'skin'). bm
Godebog MW. Guotepauc, Godebawc. May be a byname rather than a forename, from Br. *Wotepācos 'protective' < PC. *wotekʷ- 'hiding place, shelter'. hg, yg
Gogawn, Gwgawn MW. Guocaun, Gwgawn. Br. *Wocānos 'glory' (W. gogawn 'glory, honour; satisfaction'). hg, yg, t
Gorthebyr MW. Guortepir. Br. *Worteporīx, perhaps meaning 'protective king' with the same root as PC. *wotekʷ- 'hiding place, shelter'. hg
Gruffudd MW. Gripiud, OC. Grifiud. Of uncertain origin and meaning. The second element is certainly Br. *jüd- 'lord'. The first may be lL. griphus < grips 'griffin'. hg, bm
Gwaednerth MW. Waetnerth. Meaning 'blood-strength' (W. gwaed 'blood', nerth 'strength'). yg
Gwallog MW. Guallauc. Probably from Br. *Wellācos from wello- ‘better’ (W. gwell). hg, t
Gwen (f.) MW. Gwen. Br. *windā 'white' (W. gwen). yg
Gwendolen (f.) Properly *Gwenddolen from W. gwen 'white' and dolen 'ring'.
Gwener (f.) E. Venus. From L. Veneris, the genitive of L. Venus.
Gwenfraith*, Gwynfrith* (f.) OC. Guenbrith. Br. *Windobriktā 'white-speckled' (C. gwynn 'white', brith 'streaked, striped'). bm
Gwengen* (f.) OC. Guencen. Br. *Windocennā 'white skin' (C. gwynn 'white', kenn 'skin'). bm
Gwengenedl* (f.) OC. Guencenedl.  Br. *Windocenetlā 'white family' (C. gwynn 'white', kenedhel 'nation'). The word 'white' is frequently used to mean 'blessed' in the Brythonic languages, so the name probably means something like 'a family blessing'. bm
Gwidion MW. Guidgen, Guitgen, Gwydyen. From Br. *Widugenjos 'tree-born' (W. gwŷdd 'trees'). The modern name ought to be *Gwyddien but has retained a partly archaic spelling. hg, yg
Gwladus (f.) MW. Gwladus, E. Gladys. Taken literally, the name is an adjective formed from the common noun W. gwlad 'country', as if it were a female form of Patrick < L. patricius 'patrician'. This doesn't appear to be a satisfactory explanation for most authorities, however.
Gwrfoddw* OC. Gurbodu. Br. *Wirobodwos 'man crow' (C. gour 'husband', OI. bodb, badb 'hooded crow'). bm
Gwrgant* OC. Gurcant. From Br. *Wirocantos 'man circle' or 'man host' (C. gour 'husband', W cant 'outer circle, rim; hoop' or W. cant 'troop, host', OI. cét 'troops'). bm
Gwrgi, Gwrgwn* MW. Gurci, Gurgi, OC. Gurci, Gurcon. From Br. *Wirocī 'man-hound'. OC. Gurcon is from the oblique stem of the same name (Br. *Wirocunam (acc.) > W. *Gwrgwn). The natural outcome of the nominative would be W. *Gwryg, *Gwyryg, so the elements must have been reanalysed after the Brythonic period in order to retain the meaning of the name. hg, bm, bgg
Gwrhaearn MW. Gurhaiernu. Br. *Wiro-isarnos 'iron man' (W. gŵr 'man', haearn 'iron'). hg
Gwrien MW. Gwryen. Br. *Wirogenos 'man-born' (W. gŵr 'man'). yg
Gwrwared* OC. Gurguaret. Seems to be Br. *Wiroworetos 'saviour' (C. gour 'husband', W. gwared 'deliverance, salvation, help', gwaredwr 'saviour'). bm
Gwrwst MW. Gurgust, Gorust, G. Fearghas. Two similar names have probably converged here: PC. *Worgustos ‘great force’ or ‘best choice’ gives the rare OI Forggus whilst PC. *Wirogustos ‘man-force’ or ‘man-choice’ gives OI. Fergus (G. Fearghas). W. Gwrwst may be from either. hg, bgg, bs
Gwrwystl* OC. Gurguistl. Br. *Wirogēstlos 'man hostage' (C. gour 'husband', gostel 'hostage'). bm
Gwyddgwn MW. Guitcun. Br. *Widucunam (acc.) 'tree hound' (W. gwŷdd 'trees', ci 'dog'). hg
Gwyddno MW. ?Guipno (sic.), Guydno. Probably Br. *Widognāwos 'knowing trees' or 'tree-born' (W. gwŷdd 'trees') but Br. *Weidognāwos ‘knowing vision’ (W. gwybod ‘to know’, gŵydd ‘presence’) is possible. hg, bgg
Gwylog MW. Guilauc. Perhaps meaning 'watchful, vigilant' from W. gŵyl 'feast, holiday; watch, guard, vigil'. Alternatively from W. gŵyl 'modest, tender, kind, generous, glad' (G. fial 'generous, benign'). hg
Gwyn MW. Gwynn. Br. *Windos 'white' (W. gwyn).
Gwyndeyrn* OC. Guentigirn. Presumably Br. *Windotegernos 'white king' (C. gwynn 'white', myghtern 'king', W. teyrn 'king'). bm
Gwynllyw* MW. Guynlleu. Probably Br. *Windoluwī 'fair or blessed ruler' (W. gwyn 'white', llyw 'ruler'). bs
Gwynwiw* (f.) OC. Guenguiu. Identical to the Middle Welsh adjective gwynwiw 'white and fair', from W. gwyn 'white' and gwiw 'fitting, proper, fair, handsome, good'. bm
Hedrek Br. *Sitrācos 'strong or bold one' (W. hydr 'brave, bold, strong').
Heiddyn MW. Heidyn, OC. Hedyn. From Br. *sesjo- 'barley' (W. haidd) with the diminutive ending Br. *-innos (W. heiddyn 'a grain of barley'). bm, yg
Hoywgi MW. Hoewgir (sic.). Meaning 'lively dog' (W. hoyw 'alert, agile, lively', ci 'dog'). yg
Hyfaidd MW. Himeyt. Probably Br. *Sumedgos, *Subedgos 'very daring' (W. baidd 'daring'). hg
Hywel MW. Higuel, B. Hoël. From Br. *Suwelos 'well seen' in the sense of 'respectable' (W. gwelaf 'I see'). hg
Iarnwallon*, Haearnwallon* OC. Iarnguallon. Br. *Isarnowallānos 'iron leader' (C. horn 'iron'). bm
Iago W. Iago, C. Jago, E. Jacob, James. From L. Iacobus from Hebrew. bs
Idnerth MW. Iudnerth, OC. Iudnerth. Br. *Jüdonertos 'lord of strength' (MW. udd 'lord', W. nerth 'strength'). hg
Idris MW. Iudris. Uncertain. The first element is clearly Br. *jüd- 'lord' and the second may be Rhys (q.v.). The name may therefore mean 'glorious lord' or 'heroic lord'. hg
Idwawl MW. Iutguaul. From Br. *Jüdowālos, probably 'lord of light' (W. gwawl 'light, brightness, splendour'). W. gwawl also means 'wall, rampart; boundary', so the name may mean 'march lord'. hg
Ieuaf MW. Iouab. A by-name meaning 'junior' from W. ieuaf 'youngest'. hg
Ieuan, Ioan, Ifan, Iwan MW. Yeuan, B. Yann, C. Jowan, E. John. From L. Iōhannes from Hebrew. yg
Ieuenol MW. Iouanaul, E. Juvenal. From L. Iuvenālis meaning 'youthful'.  hg
Illtud B. Iltud. Probably Br. *Elutütos 'many people' (B. tud 'people, folk').
Ithael MW. Ithael. From Br. *Jüdosagelos 'generous lord' (W. udd 'lord', hael 'generous).
Jennifer (f.) W. Gwenhwyfar, E. Guinevere. From Br. *Windosɛ̄barā 'white spectre' (C. gwynn 'white', OI. síabair 'spectre, phantom').
Kadlaouen* OB. Catlouen. Probably 'battle joy' (B. kad 'fighting, combat', laouen 'joy').
Kadored OB. Catuuoret. Meaning 'battle protection' from B. kad 'fighting, combat' and gwared 'protection'.
Kadvuz OB. Catbud. Br. *Catubüdos 'battle victory' (B. kad 'fighting, combat', buz 'profit').
Kanevet OB. Catnimed. From Br. *Catunemetos, the first element is B. kad 'fighting, combat' and the second is neved 'sanctuary'.
Katouarn OB. Cathoiarn. 'Battle iron' from Br. *Catu-isarnos (B. kad 'fighting, combat', houarn 'iron').
Kenal From Br. *Cunosagelos 'generous hound' (C. ki 'dog', hel 'generous').
Keneder Br. *Cunositros 'bold hound' (C. ki 'dog', W. hydr 'bold, brave, strong').
Kenver Br. *Cunomāros 'great hound' (C. ki 'dog', meur 'great').
Lleënog* MW. Leennauc. This would seem to be the same as W. llenog 'priestly, literate, learned; scholar, cleric', from W. llên 'literature, learning; clerical' < L. *legenda 'to be read'. bs
Lleuddin* MW. Leudyn, Leudun. Probably from Br. Lugudīnos 'fort of the God Lugus' (W. Lleu 'Lugus', din 'fort'). Lleuddin is the eponymous founder of Lothian or Lleuddiniawn 'territory of Lleuddin', so the name may actually be that of a place originally. The element din does occur in other personal names, however, and the suffix -iawn, -ion is attached to personal names as in W. Ceredigion 'Cardigan'. bs
Llywarch MW. Llywarch, OC. Loumarch. Br. *Lugumarcos 'horse of the god Lugus' (W. Lleu 'Lugus', march 'horse'). bm, bgg, yg, t
Llywelyn From Br. *Lugubelinos, a combination of the divine names Lugus (W. Lleu) and Belenos.
Llywri MW. Llywri. Probably Br. *Lugurīgam (acc.) 'king of the god Lugus' (W. Lleu 'Lugus', rhi 'king'), but perhaps the first element is W. llyw 'rudder; ruler'.
Macsen, Maxen MW. Maxim, Maxen, C. Massen. From L. Maximus (in particular the Roman emperor Magnus Maximus, known as Macsen Wledig in Welsh). The name was evidently recorded at an early date according to its Latin origins and the modern Welsh name is a learned, phonetic rendering of the medieval written form. If the name had continued in use it would have become W. *Meisyf. hg, bgg, t
Madien MW. Madyen. Br. *Matugenos 'bear-born' (OI. math 'bear'). yg
Madog MW. Madawc. From Br. *Matācos, either meaning 'bear-like' (OI. math 'bear') or 'good' (W. mad 'good, virtuous, beneficial'). yg
Mael Br. *Maglos 'prince' (W. mael 'prince, chieftain, lord').
Maelgwn MW. Mailcun, Maelgun. Br. *Maglocunam (acc.) 'hound prince' (W. mael 'prince, chieftain, lord', ci 'dog'). hg, bs
Maelgwyn Br. *Maglowindos 'white prince' or 'blessed prince' (W. gwyn 'white; blessed', mael 'prince, chieftain, lord'). t
Maelog OC. Maeloc. Br. *Maglācos 'princely' (W. mael 'prince'). bm
March OC. March. From the common noun meaning 'horse' (W. march, C. margh). bm, t
Marchell (f.) MW. Marchell. From L. Marcella < Marcus. jc
Marchlew MW. Marchlew. Br. *Marcolewū 'horse lion' (W. march 'horse', llew 'lion'). The second element may be W. glew 'bold; hero'. yg
Maredudd MW. Margetiud, Morgetiud, E. Meredith. Of uncertain origin, the final element is Br. *jüd- 'lord'. hg
Marro MW. Marro. Of uncertain origin, but perhaps a hypocoristic of a name in March-. yg
Mawn, Maun MW. Maun. Uncertain, but potentially related to the divine name Mogons meaning 'great', or to the epithets of St Patrick Magonus, Mauonius, Maun, which may be from the same source or from PC. *magu- 'servant' (C. mowes 'girl', MW. meudwy 'hermit'). hg
Meddwystl* (f.) OC Medguistl. The second element is clearly C. gostel, W. gwystl 'hostage', but the first may be either C. medh, W. medd 'mead' or W. medd 'power, authority'. bm
Meirchion MW. Merchianum, Meirchaun. From L. Marciānus, a derivative of Martius, Marcius. hg, bgg, t
Meirion MW. Meriaun, Meiryaun. From L. Mariānus, a derivative or Marius. hg, bs
Merfyn MW. Mermin, OC. Mermin, E. Mervin, Mervyn, Marvin. A raft of explanations have been given for this name, none of which are particularly satisfactory. The only reliable source available to me gives the curious 'eminent marrow' from W. mêr 'marrow (literal and figurative)' and *myn 'eminent', the latter of which does not occur in the Geiriadur Prifysgol Cymru so is questionable. Other suggestions such as 'sea friend', 'sea lover' and 'sea hill' obviously take the first element to be W. môr 'sea' but have inexplicable second elements. Môr is the only reasonable candidate for the first element and it occurs in several other Brythonic names. The second element is less certain, but W. myn 'desire, wish, will' or myn 'crown, diadem' would both fit. hg, bm
Merin MW. Merin. Probably Br. *Morīnos 'sea-like' or *Morignos 'little sea' (W. môr 'sea'). yg
Merwydd MW. Merguid. Br. *Moriwidus 'sea tree' (W. môr 'sea', gwŷdd 'trees'). hg
Meurig MW. Mouric. L. Mauricius < Maurus 'dark-skinned, Moorish'. hg
Modred OC. Medrod, W. Medrawd, E. Mordred. Probably from Br. *Mātrātos or the like < *Mātr- 'mother'. bm
Môr MW. Mor, OC. Mor. Simply 'sea' (W. môr 'sea'). hg, bm
Mordaf MW. Mordaf. Perhaps Br. *Mārotamos meaning 'the greatest'. bgg, t
Morfael MW. Mormayl. Br. *Morimaglos 'sea prince' (W. môr 'sea', mael 'prince, lord'). hg
Morgan MW. Morcant, OC. Morcant. Br. *Moricantos 'sea circle' (W. môr 'sea', cant 'outer circle, rim; hoop') or 'sea host' (W. cant 'troop, host', OI. cét 'troops'). hg, bm, t
Morial MW. Moryal. Br. *Morigalos 'power of the sea' (W. môr 'sea', gallaf 'I am able'). yg
Morien MW. Morgen, Moryen.  Br. *Morigenos 'sea-born' (W. môr 'sea'). hg, yg
Morudd MW. Moriud. Br. *Morijüd- 'sea lord' (W. môr 'sea', udd 'lord'). hg
Mynog MW. Mynawc.  Br. *Monācos 'noble' (W. mynog 'noble, eminent; lord'). yg
Myrddin B. Merzin, E. Merlin. Br. *Moridīnon 'sea fort' (W. môr 'sea', din 'fort'). The original name for Carmarthen (W. Caerfyrddin), but possibly also a personal name since both elements are known in other Brythonic names.  
Neithon MW. Neithon, Nwython, Pictish Nechtan, G. Neachdainn. Of unknown origin, but perhaps originally a divine name like Br. *Nektonos, related to L. Neptūnus 'Neptune'. hg, yg
Nudd MW. Nud. From the divine name Br. *Nüdens. bgg, t, bs
Nyfed MW. Nyuet. From the common noun Br. *nemeton 'sacred place' or the adjective *nemetos 'noble'. yg
Owain MW. Ywein, Ewein, Eugein, Ouen, Owein, B. Ewen, E. Owen, Ewan, G. Eòghann. Either from PC. *Esugenjos 'born of the god Esus' or Gk. Ευγενιος (Eugenios) 'well born'. hg, yg, t, bs
Padarn MW. Patern. From L. Paternus 'paternal'. hg
Pasgen MW. Pascent. Of uncertain origin, probably derived from L. Pascha 'Easter' (W. Pasg) or L. pāscō 'I feed, foster, keep, cherish; graze' (W. pasg 'fattened, pasgaf 'I feed, fatten, nourish'). The present participle L. pascentem (acc.) 'feeding, nourishing, cultivating' may be the direct origin of the name, perhaps with Christian overtones of spiritual nourishment.  hg
Pedr MW. Petr, Pedyr, E. Peter. From L. Petrus < Gk.  Πέτρος (Petros) meaning 'stone, rock'. hg, bs
Peithan MW. Peithan. From Br. *pekto- 'strength'.
Peredur MW. Peretur, Pheredur, Peredur. no serious attempts seem to have been made to explain this name; it may contain the L. suffix –tōrius. hg, bgg, yg, t
Rhiainfellt (f.) A compound of W. rhiain 'maiden' (originally 'queen' from the same root as W. rhi 'king') and mellt 'lightning'. 
Rhian (f.) An alteration of W. rhiain 'maiden'.
Rhiannon (f.) From Br. *Rīgantonā 'divine queen' (W. rhiain 'maiden').
Rhiol* OC. Riol. Probably Br. *Rīgālos 'kingly' (C. riel 'royal', W. rhi 'king'). bm
Rhiwallon MW. Rhiwallawn.  Br. *Rīgowallānos 'king leader' (W. rhi 'king'). t
Rhodri MW. Rotri.  Perhaps Br. *Rātorīgam (acc.) 'king of troops' (W. rhawd 'course; troop', rhi 'king') or *Rotorīgam (acc.) 'wheel king' (W. rhod 'wheel'). hg
Rhufawn MW. Rumaun, Ruvawn. L. Rōmānus meaning 'Roman'. hg, yg, t
Rhun MW. Run.  Perhaps PC. *roino- 'hill, plain' (G. raon 'plain, field'). hg, t, bs
Rhydderch MW. Riderch, Ryderch. Often said to mean 'very red' or 'reddish-brown' in connection with OI. derg 'red' (G. dearg), but the root which gives OI. derg would yield *dery in Welsh. The origin is in Br. *Roderkos, connected with OI. derc 'eye', dercaim 'I behold', Gaul. derco- 'eye', so the name means 'well-seen, remarkable' or something similar. hg, bgg, t
Rhys MW. Rys. Related to W. rhyswr 'hero, champion, warrior' and rhysfa 'attack, assault, combat'. yg
Riwal Br. Rīgowalos 'king leader' (B. ri 'king').
Sawyl MW. Samuil, Sawyl. From L. Samuēl from Hebrew. hg, bgg, bs
Seisyll, Seisyllt MW. Seissil. From L. *Saxillus < Saxō 'Saxon'. hg
Selyf MW. Selim, E. Solomon. From L. Salomō from Hebrew. hg
Serwan MW. Serguan, Seruan, E. Serf, Servan. From L. Servanus < servus 'servant'.  hg, bgg, t
Serwil MW. Serguil.  L. Servīlius < Servius. hg
Siân (f.) E. Jane. Borrowed from OF. Jehanne via Middle English.
Tanghwystl, Tangwystl (f.) MW. Tancoystl, OC. Tanguistl Br. *Tancogēstlā 'peace hostage' (W. tanc 'peace, truce', gwystl 'hostage'). hg, bm
Taliessin MW. Talyessin. Br. *Talojastīnos 'radiant brow' (W. tâl 'end, forehead', iesin 'fair, beautiful; radiant, shimmering, bright'). yg, t
Tegid MW. Tacit. From L. Tacitus 'unsaid'. hg
Teilio MW. Teilyau. A hypocoristic (pet name) of *Eiludd with the pronoun ty 'your' prefixed. bs
Tewdrig MW. Tewdric. A borrowing from OE. Þeodric.
Tewdwr MW. Teudebur, W. Tewdwfr, E. Tudor, Theodore. An attempt to render L. Theodorus < Gk. Θεοδωρος using Welsh elements. The name would literally mean 'fat water'. hg
Tudfwlch MW. Tutuwlch. A compound of W. tud 'people, tribe, nation, family' and bwlch 'cleft, mountain pass', the symbolism of which is not entirely clear. bgg, yg
Tudwal MW. Tudgual, Tutagual, Tutwal. Br. *Tütowalos 'people leader' (W. tud 'people, tribe, nation, family'). hg, bgg, t
Twrch MW. Twrch. From the common noun W. twrch 'boar'. yg
Unwst OC. Ungust, G. Aonghas, E. Angus. From PC. *Oinogustus 'one force'. bm
Urien MW. Urbgen, Yrien, Uryen. Probably from Br. *Orbjogenos 'heir born' (OI. orbae 'legacy, inheritance' < PC. *orbjo-), which regularly yields MW. Yrien. The origin of the U- has been ascribed to rounding caused by the original -b-, or to a lengthening of the initial *O- > *Ō- > U-. Alternatively, U- may have begun as an attempt to write the rounded front vowel created by i-affection of o, with some scribes retaining this spelling even after the pronunciation had merged with Y-. hg, bgg, t, bs
Yannig A diminutive of B. Yann 'John'.
Yezekael OC. Iudicael, E. Judicael. Probably Br. *Jüdicosagelos 'generous lord'. The Br. stem *jüd- may derive from L. iūdex 'judge'; if so, this name appears to preserve the original consonant stem of a Br. *jüdicam (acc.), whereas other names with *jüd- appear to show that it was reanalysed as a vowel stem such as *jüdos.  

Notes

  1. The Br. *jüd- may derive from L. iūdex 'judge' or it may be native PC. *joud-. As a common noun it appeared in Middle Welsh as udd 'lord' and it became a common element in personal names.