dumb as a bag of hammers
rock-dumb
dumber than a rock
retarded
shit-for-brains
thick as a whale omelette
thick as pig shit
dumb as a stump
dumb as a post
dumb as dirt
dumb
dumb as a doorknob
dumber than a box of rocks
thick as a plank
dumb as a box of rocks
thick as two short planks
bee-brained
thick as mince
stupidass
dumb as nails
fuckwittedness
numbskulled
retardation
dumbassed
simpleminded
stupidly
pillocky
fuckwit
shitwit
bodoh
stupidy
booger brain
blockheadish
foppery
stupidicy
dumbfuckery
fatheadedly
stockishly
opaque
dumb as a rock
one brick short of a full load
retardical
dumbbutt
thick
numbnut
dumb as an ox
witless wonder
bull
mongtard
hash