dull
dim
unbright
gormless
unintelligent
nonintelligent
blockish
underwitted
not the sharpest tool in the shed
not the sharpest tool in the box
not the sharpest knife in the drawer
lunky
slow-witted
half-witted
doltish
cloistered
short-witted
nonsavvy
dull-minded
lackluster
unknowledgeable
unsmart
vacancy
stupidass
subintellectual
halfwit
simpleton
dense
simple
unsmartness
submoron
underbrained
four oh four
light-witted
cerebrally challenged
dull-witted
mentally challenged
ungenius
intellectually challenged
simplicity
unintelligence
sap
simple-mindedly
uncomprehending
lackbrain
brainless
Boeotian
stun
slow on the uptake
shitbrained