chump, loser, schlemiel (also shlemiel), turkey

slouch

airhead, birdbrain, blockhead, blunderer, bonehead, bubblehead, chowderhead, chucklehead, clodpoll (or clodpole), clot [British], cluck, clunk, cuddy (or cuddie) [British dialect], deadhead, dim bulb [slang], dimwit, dip, dodo, dolt, donkey, doofus [slang], dope, dork [slang], dullard, dumbbell, dumbhead, dum-dum, dummkopf, dummy, dunce, dunderhead, fathead, galoot [slang], 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], nincompoop, ninny, ninnyhammer, nit [chiefly British], nitwit, noddy, noodle, numskull (or numbskull), pinhead, prat [British], ratbag [chiefly Australian], saphead, schlub (also shlub) [slang], schnook [slang], simp, simpleton, stock, stupe, stupid, thickhead, turkey, woodenhead, yahoo, yo-yo

anthropoid, ape, beast, boor, brute, cad, churl, clown, creep, cretin, cur, heel, louse, skunk, snake, stinker, tyke (also tike) [chiefly British], villain

booby, fool, goose, loony (also looney), lunatic, madman, nut

featherbrain, scatterbrain

rascal, rogue, scamp