Words for whey and related things in Celtic languages.
Words marked with a * are reconstructions.
| Proto-Celtic | *mezgos = whey |
|---|---|
| Gaulish | *mesgos = whey (?) |
| Old Irish (Goídelc) | medc, medg [ˈmʲeðɡ] = whey |
| Middle Irish (Gaoidhealg) | medc, medg, midc = whey, serum |
| Irish (Gaeilge) | meadhg [ˈmʲəiɡ / ˈmʲaiɡ / ˈmʲeːɡ] = whey, serum, blood-serum – also written meidhg meadhgúil = consisting of whey, watery meadhguisce = whey-water |
| Scottish Gaelic (Gàidhlig) | meadhg [mjɤːg] = whey meang [mɛŋg] = whey meòg [mjɔːg] = whey meògach = of whey, like whey, serous meug [miag] = whey mìg [miːgʲ] = whey miong [mjɤ̃ŋg] = whey miùg [mjuːg] = whey |
| Manx (Gaelg) | meaig = whey meaighagh = whey-faced |
| Proto-Brythonic | *meið = whey |
| Middle Welsh (Kymraec) | medd, maidd, meid = whey meiddlyn = whey, buttermilk, posset |
| Welsh (Cymraeg) | maidd [mai̯ð] = whey, serum, curds and whey, blood serum meidda = to beg for whey meiddaidd, meiddl(l)yd = whey-like, wheyey, serous meiddioni = to curdle meiddlyn = whey, buttermilk, posset |
| Middle Cornish (Cernewec) | meith = whey |
| Cornish (Kernewek) | meydh = whey |
| Old Breton (Brethonoc) | meid = whey |
Note: in Breton, whey is laezh-bihan (‘small milk’) – a calque of French petit-lait (whey), laezh-glas (‘blue/green milk’), gwipad or gwitod [source] and [source].
Etymology: from Proto-Indo-European *mosgʰos (marrow, brain). The Old French word mesgue (whey) comes from the same Proto-Celtic root, via Gaulish *mesgos [source].
Words from the same PIE roots include smadzenes (brain) in Latvian, भेजा (bhejā – brain, mind) in Hindi, মজ্জা (mojja – marrow) in Bengali, مغز (maġz – brain, marrow) in Persian, marrow in English, merg (marrow) in Dutch, Mark (marrow, pith) in German, and possibly mysa (whey) in Icelandic [source].
Just in case you’re wondering, whey [weɪ] is the liquid remaining after milk has been curdled and strained in the process of making cheese. It comes from Middle English whey(e) (whey), from Old English hwǣġ (whey), from Proto-West Germanic *hwaij (whey), perhaps from Proto-Indo-European *kʷey- (to pile up, build) [source]. It can be used to be used to make products like whey cheeses, such as ricotta, whey butter, and whey protein, which is sold as a dietary supplement popular with athletes and bodybuilders. [source].
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

I had never heard the English word “whey”.
In my native language, Swedish, it would be “vassle”, which problably is related
to “water”.