not the sharpest knife in the drawer
not the sharpest tool in the box
not the sharpest tool in the shed
vacancy
unintelligence
unskillfulness
brainpower
dysrationalia
subnormality
nonintelligence
dimwit
simple-mindedness
inability
cerebricity
subnormal
sophomania
incapacity
simpleton
imbecility
simple
psychodegradation
fatuity
nondiligence
inadequacy
skillessness
feeblemindedness
stupidass
noetic
intelligentness
capacity
inexpertness
mentally deficient person
room-temperature IQ
noesis
insufficiency
foresightlessness
dyscompetence
unpower
mindpower
brilliance
disorganization
untalentedness
genius
subintellectual