moron, half-wit, imbecile, changeling, retard, cretin, nidiot, lackbrain, fool, idiota, more...

genius, smart, intelligent

moron, imbecile, dunce, numbskull, dolt, dumbass, dummy, more...

such, complete, little, old, stupid, poor, young, total, perfect, bloody, congenital

silly person,   foolish person,   dumb person,   a fool,   ignorant person,   a stupid person,   annoying doofus,   bloody fool,   senseless person,   stupid oaf