stupidity

brainlessness, denseness, density, dim-wittedness, doltishness, dopiness, dullness (also dulness), dumbness, fatuity, foolishness, mindlessness, oafishness, obtuseness, senselessness, simpleness, slowness, stupidness, thickness, vacuity, witlessness

absurdity, asininity, fatuity, folly, foolery, foppery, idiocy, imbecility, inanity, insanity, lunacy

balderdash, baloney (also boloney), beans, bilge, blarney, blather, bosh, bull [slang], bunk, bunkum (or buncombe), claptrap, drivel, folderol (also falderal), folly, foolishness, fudge, hogwash, hokeypokey, hokum, hooey, humbug, humbuggery, jazz, malarkey (also malarky), moonshine, nonsense, piffle, poppycock, rot, rubbish, senselessness, silliness, tommyrot, trash, trumpery, twaddle