The Collaborative International Dictionary of English v.0.48:

Incongruous \In*con"gru*ous\, a. [L. incongruus. See In- not,
   and Congruous.]
   Not congruous; reciprocally disagreeing; not capable of
   harmonizing or readily assimilating; inharmonious;
   inappropriate; unsuitable; not fitting; inconsistent;
   improper; as, an incongruous remark; incongruous behavior,
   action, dress, etc. "Incongruous mixtures of opinions." --I.
   Taylor. "Made up of incongruous parts." --Macaulay.
   [1913 Webster]

         Incongruous denotes that kind of absence of harmony or
         suitableness of which the taste and experience of men
         takes cognizance.                        --C. J. Smith.
   [1913 Webster]

   Incongruous numbers (Arith.), two numbers, which, with
      respect to a third, are such that their difference can not
      be divided by it without a remainder, the two numbers
      being said to be incongruous with respect to the third;
      as, twenty and twenty-five are incongruous with respect to
      four.

   Syn: Inconsistent; unsuitable; inharmonious; disagreeing;
        absurd; inappropriate; unfit; improper. See
        Inconsistent. -- In*con"gru*ous*ly, adv. --
        In*con"gru*ous*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

incongruous
    adj 1: lacking in harmony or compatibility or appropriateness;
           "a plan incongruous with reason"; "incongruous behavior";
           "a joke that was incongruous with polite conversation"
           [ant: congruent, congruous]

Moby Thesaurus II by Grady Ward, 1.0:

158 Moby Thesaurus words for "incongruous":
   abnormal, absonant, absurd, amusing, anomalous, assorted, at odds,
   at variance, bad, bizarre, clashing, colliding, conflicting,
   contradictory, contrary, contrary to reason, contrasted,
   contrasting, departing, deviating, deviative, different,
   differentiated, differing, disaccordant, disagreeing, disconsonant,
   discordant, discrepant, discrete, discriminated, disharmonious,
   disjoined, disparate, disproportionate, dissimilar, dissonant,
   distinct, distinguished, divergent, diverging, divers, diverse,
   diversified, droll, eccentric, fallacious, faulty, flawed, funny,
   futile, harsh, heterogeneous, hilarious, humorous, ill-advised,
   ill-considered, ill-suited, ill-timed, illogical, impolitic,
   improper, in disagreement, inaccordant, inadvisable, inappropriate,
   inapt, inauthentic, incoherent, incommensurable, incommensurate,
   incompatible, inconclusive, incongruent, inconsequent,
   inconsequential, inconsistent, inconsonant, inept, inexpedient,
   infelicitous, inharmonious, inopportune, invalid, irrational,
   irreconcilable, laughable, loose, ludicrous, mal a propos,
   malapropos, many, motley, multifarious, nonscientific,
   not following, off-color, off-tone, out of keeping, out of line,
   out of place, out of proportion, out of step, oxymoronic,
   paradoxical, paralogical, poles apart, poles asunder, priceless,
   quaint, quizzical, reasonless, rich, ridiculous, risible,
   screaming, self-annulling, self-contradictory, self-refuting,
   senseless, separate, separated, several, unapt, unauthentic,
   unbecoming, unbefitting, unconformable, unconnected, undesirable,
   unequal, unfit, unfitting, unfortunate, unhappy, unlike, unmeet,
   unphilosophical, unprofitable, unreasonable, unscientific,
   unseasonable, unseemly, unsuitable, unsuited, untimely, unwise,
   variant, varied, variegated, various, varying, whimsical,
   widely apart, without reason, witty, worlds apart, wrong