plagiarism (n.)

"the purloining or wrongful appropriation of another's ideas, writing, artistic designs, etc., and giving these forth as one's own" [Century Dictionary], 1620s, from -ism + plagiary (n.) "plagiarist, literary thief" (c. 1600), from Latin plagiarius "kidnapper, seducer, plunderer, one who kidnaps the child or slave of another," used by Martial in the sense of "literary thief."

This is from plagiare "to kidnap," plagium "kidnapping," which are from plaga "snare, hunting net" (also "open expanse, territory"), which is perhaps [Watkins] from PIE *plag- (on notion of "something extended"), variant form of root *plak- (1) "to be flat." De Vaan tentatively compares Greek plagia "sides, flanks," Old High German flah "flat," Old Saxon flaka "sole of the foot."