1640s, from Latin impervius "not to be traverse, that cannot be passed through, impassible," from assimilated form of in- "not, opposite of" (see in- (1)) + pervius "letting things through, that can be passed through," from per "through" (from PIE root *per- (1) "forward," hence "through") + via "road" (see via (adv.)). Related: Imperviously; imperviousness.
