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mid-14c., "authoritative," from Old French autentique (13c., Modern French authentique) "authentic; canonical," and directly from Medieval Latin authenticus, from Greek authentikos "original, genuine, principal," from authentes "one acting on one's own authority," from autos "self" (see auto-) + hentes "doer, being," from PIE *sene- "to accomplish, achieve." Sense of "entitled to acceptance as factual" is first recorded mid-14c.
Traditionally (at least since the 18c.), authentic implies that the contents of the thing in question correspond to the facts and are not fictitious; genuine implies that the reputed author is the real one; though this distinction is not etymological and is not always now recognized.