"the purloining or wrongful appropriation of another's ideas, writing, artistic designs, etc., and giving them forth as one's own," 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," from plagiare "to kidnap," plagium "kidnapping," from plaga "snare, hunting net" (also "open expanse, territory"), which is perhaps 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."
"an act of fraud, a swindle," 1969, from verbal phrase rip off "to steal or rob" (c. 1967) in African-American vernacular, from rip (v.) + off (adv.). Rip was prison slang for "to steal" since 1904, and was also used in this sense in 12c. The specific meaning "an exploitative imitation" is from 1971, also "a plagiarism." Related: Ripped-off.
c. 1600, "to shut or confine in a crib," from crib (n.). Meaning "to steal" (1748) originally was thieves' slang, probably from the noun in a secondary sense of "a basket."
This also is the probable source of student slang meaning "plagiarize; translate by means of a 'crib' " (1778). Crib (n.) in the sense of "literal translation of a classical author for illegitimate use by students" (often a Greek work rendered word-for-word into Latin) is from 1827. The meaning "something taken without permission, a plagiarism" is from 1834. Related: Cribbed; cribbing.