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], derisible

See also: crackpot [informal], crazy [informal], daft [Brit, informal], dumb, foolish, half-baked [informal], imbecile, imbecilic, moronic [informal], nutty [informal], screwball [informal], softheaded, stupid, wiggy [US, informal]