not the sharpest tool in the shed
not the sharpest tool in the box
not the sharpest knife in the drawer
stupid
underbrained
unintelligence
unintelligent
lackluster
gormless
dimwit
stupidass
half-witted
underwitted
vacancy
mentally challenged
halfwit
dull
simpleton
simple
weak
light-witted
dense
simple-mindedly
subnormality
room-temperature IQ
nonintelligence
subnormal
retard
imbecile
changeling
idiot
intellectually challenged
moron
cerebrally challenged
half-wit
cretin
feeble-minded
fuckwad
sophomania
shallow
vacant
mindless
half-brained
vacuous
dirtbrain
dysrationalia
subintellectual
indocible
unknowledgeable
nonintelligent