1. Devoid of good sense or judgment
   "imbecilic remarks";
   - foolish, dumb, stupid, daft [Brit, informal], imbecile
    
2. [archaic] Having a mental age of three to seven years
   "The patient's imbecilic behaviour required constant supervision"

See also: absurd, anserine, asinine, birdbrained [informal], blockheaded [informal], boneheaded [informal], chuckleheaded [informal], cloddish, cockeyed [informal], dense, derisible, derisory, derp [informal], dim [informal], doltish, dopey, dopy, dull, dumb, duncical [informal], duncish [informal], dunderheaded [informal], fatheaded [informal], fatuous, fond, foolish, gaumless [Brit, archaic, informal], goofy [informal], gooselike, goosey, goosy, gormless [Brit, informal], harebrained [informal], idiotic, ill-advised, ill-conceived, impolitic, imprudent, inadvisable, inane, insane, jerky, knuckleheaded [informal], laughable, loggerheaded [informal], ludicrous, lumpen, lumpish, mad, mentally retarded, mindless, misbegotten, misguided, muttonheaded [informal], nitwitted [informal], nonsensical, obtuse, pea-brained [informal], preposterous, rattlebrained, rattlepated, ridic [informal], ridiculous, scatterbrained, scatty [Brit, informal], senseless, silly, slow, soft-witted, stupid, thick [informal], thickheaded [informal], thick-skulled [informal], thick-witted [informal], unadvisable, unadvised, unintelligent, unthinking, unwise, vacant, vacuous, wacky, weak, whacky, witless, wooden-headed [informal], yokel-like, zany