inauthentic adj.

word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.

mid-14c., autentik, "authoritative, duly authorized" (a sense now obsolete), from Old French autentique "authentic; canonical" (13c., Modern French authentique) 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 root *sene- (2) "to accomplish, achieve"). The sense of "real, entitled to acceptance as factual" is recorded from mid-14c.

Traditionally in modern use, authentic implies that the contents of the thing in question correspond to the facts and are not fictitious (hence "trustworthy, reliable"); while genuine implies that the reputed author is the real one and that we have it as it left the author's hand (hence "unadulterated"); but this is not always maintained: "The distinction which the 18th c. apologists attempted to establish between genuine and authentic ... does not agree well with the etymology of the latter word, and is not now recognized" [OED].