vacancy
unintelligence
underbrained
stupid
unintelligent
lackluster
mindless
vacuous
vacant
stupidass
not the sharpest knife in the drawer
not the sharpest tool in the box
not the sharpest tool in the shed
uncomprehending
nonintelligence
mentally challenged
dense
gormless
simple
simpleton
clueless
weak
purblind
acognitive
incapacity
noncomprehension
shallow
half-brained
unbright
foresightlessness
insightlessness
inexpertness
nonintelligent
unskillfulness
intellectually challenged
cerebrally challenged
dim
nondiligence
dull
halfwit
darkness
ungenius
inexpertise
slow
unknowledgeable
incomprehension
inadequacy
inacquaintance
unperceivingness
naive