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