idiotic

airheaded, birdbrained, boobish, brain-dead, chowderheaded, chuckleheaded, clueless, dense, dim, dim-witted, doltish, dopey (also dopy), dorky [slang], dull, dumb, dunderheaded, empty-headed, fatuous, feebleminded, gormless [chiefly British], ignorant, imbecile (or imbecilic), knuckleheaded, lamebrain (or lamebrained), mindless, moronic, oafish, obtuse, opaque, pinheaded, simple, slow, slow-witted, soft, softheaded, thoughtless, thick, thickheaded, unintelligent, vacuous, witless

fallacious, illogical, invalid, irrational, nonrational, surd, unreasonable, unreasoning, unsound, weak

farcical, laughable, ludicrous, ridiculous

notional

buffoonish, clownish

ill-advised, unconsidered, unreasoned

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