nincompoops

berks [British], boobies, charlies (also charleys) [British], cuckoos, ding-a-lings, ding-dongs, dingbats, dipsticks, doofuses (also dufuses) [slang], featherheads, fools, gits [British], geese, half-wits, jackasses, lunatics, mooncalfs (or mooncalves), ninnies, ninnyhammers, nits [chiefly British], nitwits, nuts, nutcases, simps, simpletons, turkeys, yo-yos

airheads, birdbrains, blockheads, boneheads, bubbleheads, chowderheads, chuckleheads, clodpolls (or clodpoles), clots [British], clucks, clunks, cretins, cuddies [British dialect], deadheads, dim bulbs [slang], dimwits, dips, dodoes (or dodos), dolts, donkeys, doofuses (also dufuses) [slang], dopes, dorks [slang], dullards, dum-dums, dumbbells, dumbheads, dummkopfs, dummies, dunces, dunderheads, fatheads, ganders, golems, goofs, goons, half-wits, hammerheads, hardheads, idiots, ignoramuses (also ignorami), imbeciles, jackasses, know-nothings, knuckleheads, lamebrains, loggerheads [chiefly dialect], loons, lumps, lunkheads, meatheads, momes [archaic], morons, mugs [chiefly British], mutts, naturals, nimrods [slang], ninnies, ninnyhammers, nits [chiefly British], nitwits, noddies, noodles, numskulls (or numbskulls), oafs, pinheads, prats [British], ratbags [chiefly Australian], sapheads, schlubs (also shlubs) [slang], schnooks [slang], simpletons, stocks, stupes, stupids, thickheads, turkeys, woodenheads, yahoos, yo-yos