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®

preposterous
     adj : completely devoid of wisdom or good sense; "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}, {ridiculous}]