1. So unreasonable as to invite derision
   "the derisory excuse that the dog ate his homework";
   - absurd, cockeyed [informal], idiotic, laughable, ludicrous, nonsensical, preposterous, ridiculous, ridic [informal], derisible
    
2. Contemptibly small in amount
   "a derisory pay raise";
   - measly, miserable, paltry
    
3. Abusing vocally; expressing contempt or ridicule
   "He gave a derisory laugh at the suggestion";
   - derisive, gibelike, jeering, mocking, taunting, cynical

See also: crackpot [informal], crazy [informal], daft [Brit, informal], disrespectful, dumb, foolish, half-baked [informal], imbecile, imbecilic, meager [N. Amer], meagerly [N. Amer], meagre [Brit, Cdn], meagrely [Brit, Cdn], moronic [informal], nutty [informal], screwball [informal], scrimpy, softheaded, stingy [informal], stupid, wiggy [US, informal]