From Proto-Baltic *pā́ˀinas, from Proto-Indo-European *peynos, *poyHnos, from the stem *peyh₂- (“to be fat”) (perhaps from earlier “to swell”). The meaning evolved from “fat, swollen” to “(breasts) full of milk” and finally “milk”. There was an old Proto-Baltic verb *pīti (“to give milk”), from which comes dialectal Lithuanian pýti (“to give milk”); the corresponding Latvian term disappeared, perhaps because of homophony with pīt (“to braid, to weave”). Cognates include Lithuanian pi̇́enas (“milk”), Sanskrit पयते (páyate, “to swell, to be too full”), पयस् (páyas, “fluid, water, milk, rain”), Avestan 𐬞𐬌𐬞𐬌𐬌𐬏𐬱𐬌- (pipiiūši-, “having milk in her breasts”), Persian پینو (pinu, “buttermilk”), Middle Persian pēm (“milk”), Latin opīmus (“fat, plump; fruitful”).