WordWeb Online

1. Insanely irresponsible
   "an idiotic idea";
   - crackbrained
    
2. So unreasonable as to invite derision
   "the idiotic excuse that the dog ate his homework";
   - absurd, cockeyed [informal], derisory, laughable, ludicrous, nonsensical, preposterous, ridiculous, ridic [informal]
    
3. Having a mental age of three to seven years
   "I imagine that there are as many idiotic and feeble-minded women as there are imbecilic and feeble-minded men";
   - imbecilic [archaic]

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

Encyclopedia: Idiotic