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).
Literary theft. Plagiarism occurs when a writer duplicates another writer's language or ideas and then calls the work his or her own. Copyright laws protect writers' words as their legal property. To avoid the charge of plagiarism, writers take care to credit those from whom they borrow and quote.
Note: Similar theft in music or other arts is also called plagiarism.