From Proto-Italic *lawō, from Proto-Indo-European *(le-)lówh₃-ti (reduplicated o-grade present), from Proto-Indo-European *lewh₃- (“to wash”).
Cognates include Ancient Greek λούω (loúō), λοέω (loéō), Albanian laj, Old Armenian լոգանամ (loganam), and Old English lēaþor (English lather). Doublet with the later back-formed luō.