Synonyms for Preposterous:

adj.

absurd (adjective)
eccentric, bizarre, nonsensical, implausible, contradictory, absurd, senseless, unusual, incongruous, silly, illogical, inane, study at absurd, anachronistic, self-contradictory, impossible, crazy, ludicrous, paradoxical, invalid, meaningless, foolish, idiotic, outlandish, farcical, irrational, fallacious, inconsistent, loony, ridiculous.
foolish (adjective)
dippy, tomfool, zany, loopy.
insane (adjective)
balmy.
ridiculous (adjective)
outrageous, dubious, grotesque, laughable, curious, absurd, eccentric, oddball, outlandish, crazy, foolish, nonsensical, ridiculous, farcical, ludicrous, weird, bizarre, incredible, comical, poppycock, silly.
ridiculous, bizarre (adjective)
incredible, impossible, ludicrous, outrageous, insane, nonsensical, harebrained, irrational, laughable, silly, shocking, senseless, fantastic, unusual, foolish, absurd, wacky, crazy.
softheaded (adjective)
softheaded.
unbelievable (adjective)
incredible, implausible, fantastic, farfetched.

n.

over-the-top, childish, not in your right mind/not right in the head. knowledge, jerky, moronic, sappy, unearthly, dippy, balmy, harebrained, insane, tomfool, softheaded, lunatic, loony, wacky, zany, mad, loopy, ability, dopey. usual, unconscionable, obscene, shocking. preposterous (noun)
foolish, derisory, laughable, ludicrous, nonsensical, absurd, ridiculous.

Other synonyms:

loopy, softheaded, dippy, wacky, jerky, loony. harebrained, moronic. obscene, sappy, zany, unconscionable, dopey, lunatic. shocking, insane, unearthly. mad. Other relevant words:
loony, softheaded, dippy, sappy, derisory, unusual, harebrained, tomfool, absurd, lunatic, mad, moronic, jerky, loopy, obscene, insane, balmy, zany, wacky, unconscionable, shocking, unearthly, dopey, childish.

Usage examples for preposterous

  1. And if this man Hill wins his case, why it'll mean another hundred cases all as preposterous as his on our hands. – Gargoyles by Ben Hecht
  2. That there should have been two vessels of the same unusual name at one and the same time in the Port of London, was a coincidence too preposterous altogether to find place in his calculations. – The Black Bag by Louis Joseph Vance
  3. The truth was, both of us had our heads so full of mademoiselle, of Mayenne, and of Lucas, that we had forgotten the governor and his preposterous warrant. – Helmet of Navarre by Bertha Runkle