Words for king in Celtic languages.
| Proto-Celtic |
*rīxs = king |
| Celtiberian |
reikis = king |
| Gaulish |
rix = king |
| Primitive Irish |
ᚏᚔᚌᚐᚄ (rigas) = king |
| Old Irish (Goídelc) |
rí [r͈ʲiː] = king |
| Irish (Gaeilge) |
rí [ɾˠiː] = king, sovereign, monarch |
| Scottish Gaelic (Gàidhlig) |
rìgh [rʲiː] = king, Lord (God) |
| Manx (Gaelg) |
ree = king |
| Proto-Brythonic |
*riɣ [ˈriːɣ] = king |
| Old Welsh |
ri = king |
| Middle Welsh (Kymraec) |
ri = king |
| Welsh (Cymraeg) |
rhi [r̥iː] = king, prince, lord, ruler, chieftain |
| Middle Cornish |
ruy = king, sovereign ruler |
| Old Breton |
ri = king |

Etymology
From the Proto-Indo-European *h₃rḗǵs (king, ruler) [source], which is also the root of the German word Reich (empire, realm), and the English suffix -ric, as in bishopric (a diocese or region of a church which a bishop governs) [source].
The Gaulish word rix appears in names such as Vercingetorix, Dumnorix and Asterix.
| Proto-Celtic |
*brigantīnos = (someone) pre-eminent, outstanding |
| Proto-Brythonic |
*brɨɣėntin [brɨɣe̝nˈtiːn] = lord, king |
| Middle Welsh (Kymraec) |
breenhin = king |
| Welsh (Cymraeg) |
brenin [ˈbrɛnɪn / ˈbreːnɪn] = king, sovereign, monarch, head of region |
| Middle Cornish |
brentyn / bryntyn = king |
| Old Breton |
brientin / brientinion = king |
Etymology
From the Proto-Indo-European *bʰerǵʰ- (to rise; high, lofty; hill, mountain) [source],.
| Welsh (Cymraeg) |
mechdeyrn, mychdeyrn, machdeyrn = king, lord, monarch, emperor, God, tributary prince, viceroy |
| Cornish (Kernewek) |
metern [mə’tɛrn] / myghtern [mɪx’tɛrn] = king |
| Old Breton |
machtiern = king |
Etymology
From the Welsh mach (surety, guarantor, sponsor, hostage) and teyrn (sovereign, monarch, king) [source],.
Source: Gerlyver Kernewek, Geiriadur Prifysgol Cymru
| Middle Breton |
roe = king |
| Breton (Brezhoneg) |
roue = king |
Etymology
From the French roi (king), which comes, via Latin, from the Proto-Indo-European *h₃rḗǵs (king, ruler) [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
