The Collaborative International Dictionary of English v.0.48:

Absurd \Ab*surd"\ ([a^]b*s[^u]rd"), a. [L. absurdus
   harsh-sounding; ab + (prob) a derivative fr. a root svar to
   sound; not connected with surd: cf. F. absurde. See
   Syringe.]
   Contrary to reason or propriety; obviously and flatly opposed
   to manifest truth; inconsistent with the plain dictates of
   common sense; logically contradictory; nonsensical;
   ridiculous; as, an absurd person, an absurd opinion; an
   absurd dream.
   [1913 Webster]

         This proffer is absurd and reasonless.   --Shak.
   [1913 Webster]

         'This phrase absurd to call a villain great. --Pope.
                                                  --p. 9
   [1913 Webster]

   Syn: Foolish; irrational; ridiculous; preposterous;
        inconsistent; incongruous.

   Usage: Absurd, Irrational, Foolish, Preposterous. Of
          these terms, irrational is the weakest, denoting that
          which is plainly inconsistent with the dictates of
          sound reason; as, an irrational course of life.
          Foolish rises higher, and implies either a perversion
          of that faculty, or an absolute weakness or fatuity of
          mind; as, foolish enterprises. Absurd rises still
          higher, denoting that which is plainly opposed to
          received notions of propriety and truth; as, an absurd
          man, project, opinion, story, argument, etc.
          Preposterous rises still higher, and supposes an
          absolute inversion in the order of things; or, in
          plain terms, a "putting of the cart before the horse;"
          as, a preposterous suggestion, preposterous conduct, a
          preposterous regulation or law.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Absurd \Ab*surd"\ ([a^]b*s[^u]rd"), n.
   An absurdity. [Obs.] --Pope.
   [1913 Webster]

WordNet (r) 3.0 (2006):

absurd
    adj 1: inconsistent with reason or logic or common sense; "the
           absurd predicament of seeming to argue that virtue is
           highly desirable but intensely unpleasant"- Walter
           Lippman
    2: incongruous;inviting ridicule; "the absurd excuse that the
       dog ate his homework"; "that's a cockeyed idea"; "ask a
       nonsensical question and get a nonsensical answer"; "a
       contribution so small as to be laughable"; "it is ludicrous
       to call a cottage a mansion"; "a preposterous attempt to turn
       back the pages of history"; "her conceited assumption of
       universal interest in her rather dull children was
       ridiculous" [syn: absurd, cockeyed, derisory,
       idiotic, laughable, ludicrous, nonsensical,
       preposterous, ridiculous]
    n 1: a situation in which life seems irrational and meaningless;
         "The absurd is the essential concept and the first truth"--
         Albert Camus [syn: absurd, the absurd]