The Collaborative International Dictionary of English v.0.48:

Unauthentic \Unauthentic\
   See authentic.

WordNet (r) 3.0 (2006):

unauthentic
    adj 1: intended to deceive; "a spurious work of art" [syn:
           inauthentic, unauthentic, spurious]

Moby Thesaurus II by Grady Ward, 1.0:

142 Moby Thesaurus words for "unauthentic":
   Albigensian, Arian, Catharist, Donatist, Ebionitist, Erastian,
   Gnostic, Jansenist, Jansenistic, Jovinianist, Jovinianistic,
   Lollard, Manichaean, Monophysite, Monophysitic, Montanist,
   Montanistic, Pelagian, Sabellian, Waldensian, Wyclifite, absonant,
   affected, antinomian, apocryphal, artificial, assumed, bastard,
   bogus, brummagem, colorable, colored, contradictory,
   contrary to reason, counterfeit, counterfeited, distorted,
   dressed up, dummy, emanationist, embellished, embroidered,
   erroneous, ersatz, factitious, fake, faked, fallacious, falsified,
   faulty, feigned, fictitious, fictive, flawed, garbled, heretical,
   heterodox, hylotheist, hylotheistic, illegitimate, illogical,
   imitation, inauthentic, inconclusive, incongruous, inconsequent,
   inconsequential, inconsistent, invalid, irrational, junky, loose,
   make-believe, man-made, miscited, misquoted, misreported,
   misstated, mock, nonofficial, nonorthodox, nonscientific,
   not following, pantheist, pantheistic, paralogical, perverted,
   phony, pinchbeck, pretended, pseudo, put-on, quasi, queer,
   reasonless, self-annulling, self-contradictory, self-refuting,
   self-styled, senseless, sham, shoddy, simulated, so-called,
   soi-disant, spurious, supposititious, synthetic, tin, tinsel,
   titivated, twisted, unaccepted, unapproved, unattested,
   unauthenticated, unauthoritative, uncanonical, uncertified,
   unchecked, unconfirmed, unconnected, uncorroborated,
   undemonstrated, unfounded, ungenuine, unnatural, unofficial,
   unorthodox, unphilosophical, unproved, unreal, unreasonable,
   unreliable, unscientific, unscriptural, unsound, unvalidated,
   unverified, unwarranted, warped, without reason