Fool word family

fool concrete noun / verb
foolishness noun
foolish adjective
foolishly adverb

Fool idioms and compounds

Tom foolery
Play the fool
Fool around ( + with – sexual in meaning)
Fools rush in where angles fear to tread

Idiocy word family

Idiot concrete noun
idiocy noun
idiotic adjective
Idiotically adverb

Idiocy idioms and compounds

Village idiot

Retard word family

retard concrete noun / verb
retardation noun
retarded adjective

Other nouns and idioms

Twit concrete noun
Imbecile concrete noun
Daft as a brush
Soft in the head
A penny short
Not the brightest button in the box
Dumb (adj.)
Thick (adj.)
Thick as two short planks
Thick as pigshit
Dense
Knucklehead
Dunce
Dumbass
Dipstick
Dipshit
Noodlehead
Blonde
Bonehead
Brainless (adj.)
Gormless (adj.)
Bit of a Boris
Bit of a Trump