Words for nine and related things in Celtic languages:

| Proto-Celtic | *nowan = nine *naumetos = ninth |
|---|---|
| Gaulish | *nau = nine nametos = ninth |
| Old Irish (Goídelc) | noí [n͈oːi̯] = nine nómad = ninth nónbor = nine people noínden = nine days |
| Middle Irish (Gaoidhealg) | noí, noi = nine nómad, nomad = ninth nónbor = nine people noínden = nine days noíchtige = a period of 29 days nócha, nocha, nochat = ninety |
| Irish (Gaeilge) | naoi [n̪ˠiː/n̪ˠɰiː] = nine (an) naoú = ninth naonúr = nine people naoi déag = nineteen nócha = ninety naoichodach = ninefold, having nine parts |
| Scottish Gaelic (Gàidhlig) | naoi(dh) [n̪ˠɯj] = nine naodh [n̪ˠɯːɣ] = nine naoitheamh [n̪ˠɤjəv] (9ᵐʰ) = ninth (9ᵗʰ) naoinear [n̪ˠɯːn̪ʲər] = nine (people) naoi deug = nineteen naochad [n̪ˠɯːxəd] = ninety naoidh-fillte = nonuple, ninefold, nine-ply |
| Manx (Gaelg) | nuy [nɛi/niː] = nine (yn) nuyoo = (the) ninth nuy jeig = nineteen nuy-cheayrtyn, nuy-filley = ninefold nuy-uillinagh = nonagonal, nonagon |
| Proto-Brythonic | *naw [n͈oːi̯] = nine *nọβ̃ed = ninth |
| Old Welsh | naw = nine |
| Middle Welsh (Kymraec) | nav, nau, naw = nine navuet, nauuet, nawued, nawuet = ninth naw deg, naw-deg = ninety nawkan, naw cant, nawcant = nine hundred, many, numerous naw ugein(t) = 180 nawbann, nowban = (having) nine syllables (in Welsh poetry) nawwell, nawell = nine times better (than), much better naun, nawn, naon = the ninth hour of the day naw nyn, nawnyn = nine men |
| Welsh (Cymraeg) | naw [naːu̯/nau̯] = nine nawfed [ˈnau̯vɛd/ˈnau̯vad] (9fed) = ninth, one of nine, nones (in Roman calendar) deunaw = eighteen (two nines) naw deg = ninety nawcant = nine hundred, many, numerous nawban, naw ban = (having) nine syllables (in Welsh poetry) nawell = nine times better (than), much better nawn = the ninth hour of the day (approx. 3pm) midday, nooon, afternoon nawnbryd = evening meal, dinner, supper nawnyn, naw nyn = nine men nawplyg = ninefold |
| Middle Cornish (Cernewec) | naw = nine nawnzac, nawntek, nownsec, nowndzhak = nineteen naiv cans = nine hundred |
| Cornish (Kernewek) | naw = nine nawves = ninth nownsek = nineteen nowsegves = nineteenth |
| Old Breton | nau = nine |
| Middle Breton (Brezonec) | nau, nao, naou, naff, nauë, naü = nine navet = ninth nauntec, nantec, nandec = nineteen naontecvet, nandecvèd, naontekved, naoñteget = nineteenth nao ugent = 180 |
| Breton (Brezhoneg) | nav [ˈnaw] = nine navet = ninth naontek = nineteen |
Etymology: from Proto-Indo-European *h₁néwn̥ (nine) and *h₁newn̥nós (ninth) [source].
English words from the same roots include nine, and words beginning with ennea-, such as enneagon (a 9-sided polygon) and enneastyle (having 9 columns) [source].
Sources: Wiktionary, Am Faclair Beag, Online Manx Dictionary, Teanglann.ie, eDIL – Electronic Dictionary of the Irish Language, In Dúil Bélrai English – Old Irish glossary, Geiriadur Prifysgol Cymru, Gerlyver Kernewek, Gerlyvyr Cernewec, Dictionaire Favereau, TermOfis, Le dictionnaire diachronique du breton, Geriafurch, English – ProtoCeltic WordList (PDF), Etymological Dictionary Of Proto Celtic








