absurd, asinine, balmy, brainless, bubbleheaded, cockeyed, crackpot, crazy, cuckoo, daffy, daft, dippy, dotty, fatuous, featherheaded, fool, foolish, half-baked, half-witted, harebrained, inept, insane, jerky, kooky(also kookie), loony(also looney), lunatic, lunkheaded, mad, nonsensical, nutty, preposterous, sappy, screwball, senseless, silly, simpleminded, stupid, unwise, wacky(also whacky), weak-minded, witless, zany

airheaded, birdbrained, boobish, brain-dead, chowderheaded, chuckleheaded, clueless, dense, dim, dim-witted, doltish, dopey(also dopy), dorky(slang), dull, dumb, dunderheaded, empty-headed, feebleminded, gormless(chiefly British), idiotic(also idiotical), imbecile(or imbecilic), knuckleheaded, lamebrain(or lamebrained), mindless, moronic, oafish, obtuse, opaque, pinheaded, simple, slow, slow-witted, soft, softheaded, thoughtless, thick, thickheaded, unintelligent, vacuousfallacious, illogical, invalid, irrational, nonrational, surd, unreasonable, unreasoning, unsound, weakfarcical, laughable, ludicrous, ridiculousnotionalbuffoonish, clownishill-advised, unconsidered, unreasoned

out to lunch