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).