cheeser
dumbbell
dope
countling
underwitted
folly
weak-minded
unwary
unbright
chowderheaded
guileless
thickwitted
tardlet
not the sharpest tool in the shed
fool
unknowledgeable
not the sharpest knife in the drawer
sweet summer child
underimagined
not the sharpest tool in the box
gonk
cubbish
lunky
divvie
room-temperature IQ
unforesighted
unsmart
foresightless
tinhorn
bullet-headed
smarten up
immaturely
weak-mindedly
jejunely
fall off the turnip truck
starry-eyed
slow-witted
sapheaded
ingenue
fuckheaded
be born yesterday
cerebrally challenged
boofheaded
kiddish
rattlebrained
bubble-brained
intellectually challenged
infantile