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Adjective: preposterous  pri'pó-stu-rus
  1. So unreasonable as to invite derision
    "a preposterous attempt to turn back the pages of history";
    - absurd, cockeyed [informal], derisory, idiotic, laughable, ludicrous, nonsensical, ridiculous, ridic [informal]

See also: daft [Brit, informal], dumb, foolish, imbecile, imbecilic, stupid