The origin of this word is unclear, but probably Proto-Indo-European. Some compare it to Lower Sorbian para (“filth, dung”), Polabian poro (“filth, dung; swamp, bog”) (assuming that o < *uo < *ō), from a stem *pōr- to which an extra v or vo was added. Others relate purvs to Ancient Greek πυρρός (purrhós, “red, pink, brownish red”), Sanskrit पुरीषम् (púrīṣam, “filth, dung”), परुष (paruṣa, “spotted, speckled”), pointing out that it often happens in Indo-European languages that "swamp" and "red" are related terms (e.g., from Proto-Slavic *ruda (“red”), Polish ruda (“ore, peat, swamp”), Ukrainian руда́ (rudá, “ore, swamp”)), probably because of the reddish-brown color of swamp waters. Cognates include Lithuanian pur̃vas (“filth, dirt”), Sudovian puro (“swamp”).