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):

ridiculousness
    n 1: a message whose content is at variance with reason [syn:
         absurdity, absurdness, ridiculousness]