plagiarist (n.)

"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."

word-forming element meaning "one who does or makes," also used to indicate adherence to a certain doctrine or custom, from French -iste and directly from Latin -ista (source also of Spanish, Portuguese, Italian -ista), from Greek agent-noun ending -istes, which is from -is-, ending of the stem of verbs in -izein, + agential suffix -tes.

Variant -ister (as in chorister, barrister) is from Old French -istre, on false analogy of ministre. Variant -ista is from Spanish, popularized in American English 1970s by names of Latin-American revolutionary movements.