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]

Moby Thesaurus II by Grady Ward, 1.0:

147 Moby Thesaurus words for "absurd":
   Pickwickian, a bit thick, a bit thin, abnormal, amusing, anomalous,
   asinine, balmy, barred, beyond belief, bizarre, childish,
   closed-out, cockamamie, comic, contrary to reason, crazy, curious,
   daft, disproportionate, doubtable, doubtful, droll, dubious,
   dubitable, eccentric, empty, excluded, extravagant, fantastic,
   farcical, fatuitous, fatuous, foolish, freaked out, freaky, funny,
   futile, grotesque, hard of belief, hard to believe, harebrained,
   high-flown, hilarious, hollow, hopeless, humorous, idiotic, idle,
   illogical, imbecile, imbecilic, implausible, impossible, inane,
   incoherent, incommensurable, incommensurate, incompatible,
   inconceivable, incongruous, inconsequent, inconsistent,
   inconsonant, incredible, insane, irrational, irreconcilable, kooky,
   laughable, logically impossible, loony, ludicrous, mad,
   meaningless, monstrous, moronic, nonsensical, not deserving belief,
   not possible, nuts, nutty, odd, oddball, off, off the wall,
   open to doubt, open to suspicion, out, out of proportion,
   outlandish, outrageous, outre, oxymoronic, paradoxical,
   passing belief, passing strange, peculiar, poppycockish, potty,
   preposterous, priceless, problematic, prohibited, quaint, queer,
   questionable, quizzical, rich, ridiculous, risible, rubbishy,
   ruled-out, screaming, self-contradictory, senseless, silly, simple,
   singular, skimble-skamble, staggering belief, strange, stupid,
   suspect, suspicious, tall, thick, thin, trashy, twaddling, twaddly,
   unbelievable, unconvincing, unearthly, ungodly, unimaginable,
   unreasonable, unsound, unthinkable, unworthy of belief, vain,
   wacky, weird, whimsical, wild, witty, wondrous strange