From the same stem as the adjective sūrs (“bitter”) (q.v.), from which it is ultimately derived, in the (older) form *sūris or *sūrs, from Proto-Balto-Slavic *sū́ˀras; the original meaning was thus “bitter (one)”. The vowel was later changed under the influence of the common expression siet sieru “to make (lit. tie, bind) cheese”. Cognates include Lithuanian sū́ris, Old Prussian suris (sūris), sur (sūr), Russian, Belarusian сыр (syr), Ukrainian сир (syr), Bulgarian си́рене (sírene), Czech sýr, Polish ser, Old Norse sýra (“sour milk”), Old High German, Old English sūr, German sauer, English sour.