WordWeb Online

1. Devoid of good sense or judgment
   - foolish, daft [Brit, informal], dumb, stupid, imbecile
    
2. [archaic] Having a mental age of three to seven years
   "I imagine that there are as many imbecilic and feeble-minded women as there are imbecilic and feeble-minded men";
   - idiotic

See also: absurd, cockeyed [informal], derisory, fond, goofy [informal], harebrained, idiotic, ill-advised, ill-conceived, impolitic, imprudent, inadvisable, insane, laughable, ludicrous, mad, mentally retarded, mindless, misbegotten, misguided, nonsensical, preposterous, rattlebrained, rattlepated, retarded, ridic [informal], ridiculous, scatterbrained, scatty [Brit, informal], silly, unadvisable, unadvised, unwise, vacant, vacuous, wacky, whacky, zany