From Proto-Italic *plowō, from Proto-Indo-European *plew- (“to flow, float, wash”), with a metaphorical usage of “to swim, float” to refer to pouring rain; see also Latin linter (“bath, trough, boat”), Ancient Greek πλύνω (plúnō, “to wash”), Lithuanian pi̇̀lti (“to pour”) and German fließen (“to flow”).
See Old Armenian լուանամ (luanam, “to wash”).