The Collaborative International Dictionary of English v.0.48:

Ridiculous \Ri*dic"u*lous\, a. [L. ridiculosus, ridiculus, fr.
   ridere to laigh. Cf. Risible.]
   1. Fitted to excite ridicule; absurd and laughable; unworthy
      of serious consideration; as, a ridiculous dress or
      behavior.
      [1913 Webster]

            Agricola, discerning that those little targets and
            unwieldy glaives ill pointed would soon become
            ridiculous against the thrust and close, commanded
            three Batavian cohorts . . . to draw up and come to
            handy strokes.                        --Milton.
      [1913 Webster]

   2. Involving or expressing ridicule. [R.]
      [1913 Webster]

            [It] provokes me to ridiculous smiling. --Shak.
      [1913 Webster]

   Syn: Ludicrous; laughable; risible; droll; comical; absurd;
        preposterous. See Ludicrous.
        [1913 Webster] --- Ri*dic"u*lous*ly, adv. --
        Ri*dic"u*lous*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

ridiculous
    adj 1: inspiring scornful pity; "how silly an ardent and
           unsuccessful wooer can be especially if he is getting on
           in years"- Dashiell Hammett [syn: pathetic,
           ridiculous, silly]
    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]
    3: broadly or extravagantly humorous; resembling farce; "the
       wild farcical exuberance of a clown"; "ludicrous green hair"
       [syn: farcical, ludicrous, ridiculous]

Moby Thesaurus II by Grady Ward, 1.0:

98 Moby Thesaurus words for "ridiculous":
   a bit thick, a bit thin, absurd, amusing, antic, barred,
   beyond belief, bizarre, closed-out, cockamamie, comic, comical,
   contrary to reason, crazy, doubtable, doubtful, droll, dubious,
   dubitable, eccentric, excluded, extravagant, fantastic, farcical,
   foolish, funny, grotesque, hard of belief, hard to believe,
   high-flown, hilarious, hopeless, humorous, implausible, impossible,
   improper, inane, inconceivable, incongruous, incredible, indecent,
   indelicate, insane, laughable, logically impossible, ludicrous,
   malodorous, mirthful, monstrous, nonsensical, not deserving belief,
   not possible, open to doubt, open to suspicion, outlandish,
   outrageous, outre, oxymoronic, paradoxical, passing belief,
   poppycockish, preposterous, priceless, problematic, prohibited,
   quaint, queer, questionable, quizzical, rich, risible, rough,
   ruled-out, screaming, self-contradictory, silly, staggering belief,
   stupid, suspect, suspicious, tall, thick, thin, unbecoming,
   unbelievable, unconvincing, undecorous, unearthly, ungodly,
   unimaginable, unseemly, unthinkable, unworthy of belief, weird,
   whimsical, wild, witty, zany