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]