From Proto-Italic *ruðros, from Proto-Indo-European *h₁rudʰrós (“red”), from the root *h₁rewdʰ-.
Cognates include Old Armenian արուրդ/երուրդ (arurd/erurd) or արոյդ/երոյդ (aroyd/eroyd), Ancient Greek ἐρυθρός (eruthrós), Sanskrit रुधिर (rudhirá), Old East Slavic ръдръ (rŭdrŭ) (< Proto-Slavic *rъdrъ). Compare dialectal form rūfus (“reddish, ruddy”).