The Collaborative International Dictionary of English v.0.48:

Fallacious \Fal*la"cious\, a. [L. fallaciosus, fr. fallacia: cf.
   F. fallacieux. See Fallacy.]
   Embodying or pertaining to a fallacy; illogical; fitted to
   deceive; misleading; delusive; as, fallacious arguments or
   reasoning. -- Fal*la"cious*ly, adv. -Fal*la"cious*ness,
   n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fallacious
    adj 1: containing or based on a fallacy; "fallacious reasoning";
           "an unsound argument" [syn: fallacious, unsound]
    2: intended to deceive; "deceitful advertising"; "fallacious
       testimony"; "smooth, shining, and deceitful as thin ice" -
       S.T.Coleridge; "a fraudulent scheme to escape paying taxes"
       [syn: deceitful, fallacious, fraudulent]
    3: based on an incorrect or misleading notion or information;
       "fallacious hope"

Moby Thesaurus II by Grady Ward, 1.0:

171 Moby Thesaurus words for "fallacious":
   Albigensian, Arian, Barmecidal, Barmecide, Catharist, Donatist,
   Ebionitist, Erastian, Gnostic, Jansenist, Jansenistic, Jovinianist,
   Jovinianistic, Lollard, Manichaean, Monophysite, Monophysitic,
   Montanist, Montanistic, Pelagian, Sabellian, Waldensian, Wyclifite,
   aberrant, abroad, absonant, adrift, airy, all abroad, all off,
   all wrong, amiss, antinomian, apocryphal, apparent,
   apparently sound, apparitional, askew, astray, at fault, autistic,
   awry, beguiling, beside the mark, casuistic, catchy, chimeric,
   colorable, contradictory, contrary to fact, contrary to reason,
   corrupt, deceiving, deceptive, defective, deluding, delusional,
   delusionary, delusive, delusory, dereistic, deviant, deviational,
   deviative, disingenuous, distorted, dreamlike, dreamy, dubious,
   emanationist, empty, errant, erring, erroneous, false, fantastic,
   faultful, faulty, fishy, flawed, hallucinatory, heretical,
   heterodox, hollow, hylotheist, hylotheistic, illogical, illusional,
   illusionary, illusive, illusory, imaginary, in error, inauthentic,
   inconclusive, incongruous, inconsequent, inconsequential,
   inconsistent, insincere, invalid, irrational, jesuitic, loose, mad,
   misleading, nonorthodox, nonrational, nonscientific, not following,
   not right, not true, off, off the track, ostensible, out,
   overrefined, oversubtle, pantheist, pantheistic, paralogical,
   peccant, perverse, perverted, phantasmagoric, phantasmal, phantom,
   philosophistic, plausible, questionable, reasonless, seeming,
   self-annulling, self-contradictory, self-deceptive, self-deluding,
   self-refuting, senseless, sophistic, sophistical, specious,
   spectral, straying, supposititious, trickish, tricksy, tricky,
   truthless, unaccepted, unactual, unapproved, unauthentic,
   unauthoritative, uncanonical, unconnected, unfactual, unfounded,
   unorthodox, unphilosophical, unproved, unreal, unreasonable,
   unscientific, unscriptural, unsound, unsubstantial, untrue,
   visionary, wide, without reason, wrong