thick as mince
stupidass
dumb as dirt
dumb
dumb as a post
thick as pig shit
thick as a whale omelette
bee-brained
rock-dumb
shit-for-brains
dumber than a rock
dumb as a stump
retarded
thick as two short planks
thick as a plank
dumber than a box of rocks
dumb as a doorknob
dumb as a box of rocks
dumb as a bag of hammers
dumb as nails
bodoh
blockheadish
stupidy
booger brain
shitwit
fuckwit
pillocky
dumbassed
numbskulled
simpleminded
pratty
stockishly
fatheadedly
dopey
stupidish
opaque
dumb as an ox
dumbbutt
bull
thick
one brick short of a full load
retardical
numbnut
dumbish
dumb as a rock
thickhead
puzzlewit
dym
dumbnut
dumb like a fox