1650s, "pertaining to rain," from French pluvial (12c.), from Latin pluvialis "pertaining to rain, rainy, rain-bringing," from (aqua) pluvia "rain (water)," from fem. of pluvius "rainy," from plovere "to rain," from PIE root *pleu- "to flow, to swim" (cognates: Sanskrit plavate "navigates, swims;" Greek plynein "to wash," plein "to navigate," ploein "to float, swim," plotos "floating, navigable;" Armenian luanam "I wash;" Old English flowan "to flow;" Old Church Slavonic plovo "to flow, navigate;" Lithuanian pilu, pilti "to pour out," plauju, plauti "to swim, rinse").