dense
slow on the uptake
slow
mentally challenged
vacancy
unintelligence
underbrained
half-brained
not the sharpest tool in the box
dullness
not the sharpest tool in the shed
not the sharpest knife in the drawer
dumb
unintelligent
stupid
obtuse
dull
dim
obtuseness
blunt
half-witted
shallow
subexpert
stupidass
intellectually challenged
cerebrally challenged
subnormal
fool
subnormality
noncomprehension
idiot
gormless
nonintelligence
blockish
halfwit
tubelight
uncomprehending
subintellectual
unbright
clueless
dirtbrain
dozy
unskillfulness
weak
lackluster
nondiligence
talk fast and loose
thick as a brick
inexpertise
nonunderstanding