Words for first, before, previous, and related things in Celtic languages.

Words marked with a * are reconstructions.

Proto-Celtic	*kentus = first *kentuslongos = early meal *kentusamonyos, *kintusamīnos, *kintusamonis = beginning of summer, May *kentusedom = first seat
Gaulish	cintus = first (?)
Old Irish (Goídelc)	cét- [ˈkʲeː.d] = first cétaín [ˈkʲeː.dai̯nʲ] = Wednesday (“first fast”) cétmuinter = spouse, wife cétnae = first, same cétóir = first time, beginning
Middle Irish (Gaoidhealg)	cét- [kax / kex] = first, early, primary, former cétad = seat, couch cétaín [ˈkʲeː.dai̯nʲ] = Wednesday cétlongad = fasting
Irish (Gaeilge)	céad [ciːa̯d̪ˠ / ceːd̪ˠ] = first Céadaoin [ˈceːd̪ˠiːnʲ] = Wednesday céalacan = moring fast
Scottish Gaelic (Gàidhlig)	ciad [kʲiad] = first ciadaineach [kʲiədɪn̪ʲəx] = pertaining to Wednesday, falling on a Wednesday DiCiadain [dʲɪˈkʲiədɪn̪ʲ] = Wednesday
Manx (Gaelg)	kied = first, primary, senior chied = first, initial, primary kied chouyr = first aid Crean = Mercury, Wednesday
Proto-Brythonic	*kɨnt = earlier, sooner, before, former, previous *kɨntüheð = porch, entry, court (?) *kɨntühaβ̃in = beginning of summer, first month of summer, May (?)
Middle Welsh (Kymraec)	cin, kin, kyn(n), cynn = before, previous to, preceding cynda(i)d = ancestor, forefather kynt, cynt = earlier, sooner, former(ly), previous(ly), before, quicker, faster, swifter, rather (than) kyn(n)hayaf, kynhaeaf, cynhaeaf = harvest(-time), autumn kantaf, kyntaf, kyntaw, kentaf = swiftest, quickest, fastest, fleetest, first, foremost, chief, principal cyntedd = porch, lobby, vestibule, entry, court, forecourt kintevin, kynteuin, kyntefin = beginning of summer, first month of summer, May, spring, summer kythlwng = fasting, hunger
Welsh (Cymraeg)	cyn [kɨ̞n / kɪn] = before, previous to, preceding cynt [kɨ̞nt / kɪnt] = earlier, sooner, former(ly), previous(ly), before, quicker, faster, swifter, rather (than) cyntaf = swiftest, quickest, fastest, fleetest, first, foremost, chief, principal cynhaeaf = harvest(-time), autumn cyntedd [ˈkəntɛð] = porch, lobby, vestibule, entry, court, forecourt cyntefin [kənˈtɛvɪn] = beginning of summer, first month of summer, May, spring, summer cythlwng = fasting, hunger
Old Cornish	kyniaf = harvest(-time), autumn
Middle Cornish (Cernewec)	cyn, cen, ken = first, before (that) cens, kens = before, formerly, rather cenzhoha, kenzhoha = the morning cynsa, censa, kensa = the first, chief cyniaf, kyniaw, cidniadh, cidniaz = autumn, harvest, time (“before winter”)
Cornish (Kernewek)	kyns, kens = former, previous, sooner, earlier, formerly, rather, before kyns ena, kens ena = previously, before then kynsa, kensa = first, initial kynser(es) = apprentice kynserneth = apprenticeship kynsistorek = prehistoric
Middle Breton (Brezonec)	quent = before quentaff = first
Breton (Brezhoneg)	kent [ˈkɛ̃nt] = before, previously, rather, preferably, prevous, prior kantad = ancestor kantael = primitive, primary kantaenn = principle kantañ [ˈkẽn.tã] = first, main, next, soon kentannezer = pioneer, pathfinder kentanv = first name kentocʼh [ˈkẽntɔx] = rather, instead kerkent [ˈkɛrkɛnt] = immediately, as soon as

Etymology: from Proto-Indo-European *kéntus, from *ken- (to arise, begin) [Source]. Words from the same roots possibly include begin, recent in English, beginnen (to begin, commence, start) in German, cenau (cub, pup, whelp, rascal) in Welsh, cana (cub, whelp) in Irish, szczenię (puppy, cub) in Polish, and reciente (recent) in Spanish [Source].
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Sources: Wiktionary, Etymological Dictionary Of Proto Celtic, In Dúil Bélrai English – Old Irish glossary, eDIL – Electronic Dictionary of the Irish Language, Teanglann.ie, Am Faclair Beag, An etymological dictionary of the Gaelic language, Fockleyreen: Manx – English Dictionary, Online Manx Dictionary, Gaelg Corpus, Geiriadur Prifysgol Cymru, Lexicon cornu-britannicum : a dictionary of the ancient Celtic language of Cornwall, Gerlyver Kernewek, Devri : Le dictionaire diachronique du breton, Geriafurch, TermOfis