The Collaborative International Dictionary of English v.0.48:

Preposterous \Pre*pos"ter*ous\, a. [L. praeposterus; prae before
   + posterus coming after, latter. See Posterior.]
   [1913 Webster]
   1. Having that first which ought to be last; inverted in
      order. [Obs.]
      [1913 Webster]

            The method I take may be censured as preposterous,
            because I thus treat last of the antediluvian earth,
            which was first in the order of nature. --Woodward.
      [1913 Webster]

   2. Contrary to nature or reason; not adapted to the end;
      utterly and glaringly foolish; unreasonably absurd;
      perverted. "Most preposterous conclusions." --Shak.
      [1913 Webster]

            Preposterous ass, that never read so far! --Shak.
      [1913 Webster]

   Syn: Absurd; perverted; wrong; irrational; foolish;
        monstrous. See Absurd.
        [1913 Webster] -- Pre*pos"ter*ous*ly, adv.
        -Pre*pos"ter*ous*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

preposterously
    adv 1: so as to arouse or deserve laughter; "her income was
           laughably small, but she managed to live well" [syn:
           laughably, ridiculously, ludicrously,
           preposterously]