Preposterous a.
Etymology: [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.
Shades of meaning
Fernald has no entry of its own for preposterous; he lists it under absurd, queer.
Where he writes about it:
Monstrous and preposterous refer to what is overwhelmingly absurd; as, "O monstrous! eleven buckram men grown out of two," SHAKESPEARE 1 King Henry IV, act ii, sc. 4. — from absurd