Words for sea in Celtic languages:
| Proto-Celtic | *mori = sea |
|---|---|
| Primitive Irish | *ᚋᚑᚏᚔᚅ (*morin) = sea |
| Old Irish (Goídelc) | muir = sea |
| Irish (Gaeilge) | muir = sea |
| Scottish Gaelic (Gàidhlig) | muir [murʲ] = sea |
| Manx (Gaelg) | mooir = sea |
| Proto-Brythonic | *mor = sea |
| Middle Welsh (Kymraec) | mor = sea |
| Welsh (Cymraeg) | môr [moːr] = sea, ocean, the deep; plenty, abundance, copiousness |
| Old/Middle Cornish | mor = sea |
| Cornish (Kernewek) | mor = sea |
| Old/Middle Breton | mor = sea |
| Breton (Brezhoneg) | mor = sea |
Etymology: from the Proto-Indo-European móri (sea, standing water), from *mer- (sea, lake, wetland), which is also the root for the English word mere, as in Windermere [source].
In Welsh there is another word for sea: gweilgi, which also means ocean, the deep, flood or torrent. It comes from gwae (woe, great misery, affliction, distress, anguish, trouble, adversity) and ci (dog).
Sources: Wiktionary, Am Faclair Beag, teanglann.ie, On-Line Manx Dictionary, Geiriadur Prifysgol Cymru, Gerlyver Kernewek, Dictionnaire Favereau
What about ‘gweilgi’ in Welsh, too?