Source : Webster's Revised Unabridged Dictionary (1913)

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

            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.

   2. Contrary to nature or reason; not adapted to the end;
      utterly and glaringly foolish; unreasonably absurd;
      perverted. ``Most preposterous conclusions.'' --Shak.

            Preposterous ass, that never read so far! --Shak.

   Syn: Absurd; perverted; wrong; irrational; foolish;
        monstrous. See {Absurd}. -- {Pre*pos"ter*ous*ly}, adv.
        -{Pre*pos"ter*ous*ness}, n.

Source : WordNet®

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