1620s, from -ism + plagiary (n.) "plagiarist, literary thief" (1590s), from Latin plagiarius "kidnapper, seducer, plunderer, one who kidnaps the child or slave of another," used by Martial in the sense of "literary thief," from plagiare "to kidnap," plagium "kidnapping," from plaga "snare, hunting net," perhaps from PIE *plag- (on notion of "something extended"), from root *plak- (1) "to be flat" (see placenta).