nincompoop

berk [British], booby, charlie (also charley) [British], cuckoo, ding-a-ling, ding-dong, dingbat, dipstick, doofus [slang], featherhead, fool, git [British], goose, half-wit, jackass, lunatic, mooncalf, ninny, ninnyhammer, nit [chiefly British], nitwit, nut, nutcase, simp, simpleton, turkey, yo-yo

airhead, birdbrain, blockhead, bonehead, bubblehead, chowderhead, chucklehead, clodpoll (or clodpole), clot [British], cluck, clunk, cretin, cuddy (or cuddie) [British dialect], deadhead, dim bulb [slang], dimwit, dip, dodo, dolt, donkey, doofus [slang], dope, dork [slang], dullard, dum-dum, dumbbell, dumbhead, dummkopf, dummy, dunce, dunderhead, fathead, gander, golem, goof, goon, half-wit, hammerhead, hardhead, idiot, ignoramus, imbecile, jackass, know-nothing, knucklehead, lamebrain, loggerhead [chiefly dialect], loon, lump, lunkhead, meathead, mome [archaic], moron, mug [chiefly British], mutt, natural, nimrod [slang], ninny, ninnyhammer, nit [chiefly British], nitwit, noddy, noodle, numskull (or numbskull), oaf, pinhead, prat [British], ratbag [chiefly Australian], saphead, schlub (also shlub) [slang], schnook [slang], simpleton, stock, stupe, stupid, thickhead, turkey, woodenhead, yahoo, yo-yo