too big for one's boots
underdo
subexpert
wimp
pond scum
subhuman
not the sharpest tool in the box
not the sharpest tool in the shed
not the sharpest knife in the drawer
humble
simpleton
gormless
fool
simple
submoron
half-witted
worse
pig dog
underwitted
Boeotian
no-mark
crybaby
light-witted
chicken
simple-mindedly
gobemouche
in the land of the blind, the one-eyed man is king
gobe-mouche
invalorous
tubelight
cloistered
Muggle
outskilled
above oneself
pelf
out of one's league
cull
coward
unknowledgeable
lame duck
lackluster
dull
reject
lessener
God hates a coward
dummy
boob
dropkick
dipstick
booby
inadequate
dope
modesty
dumbbell
stun
pinhead
subintellectual
lightweight
timid
untalentedness
paper tiger
indiscernible
insignificancy
subnormal
if all you have is a hammer, everything looks like a nail
second best
chopped liver
insect
cerebrally challenged
innocence
divvie
jack of all trades, master of none
naïveté
intellectually challenged
unlearned
diffident
indistinguishable
incognoscible
inability
wannabe
incognizable
characterless
legend in one's own lunchtime
gutless
noninvincibility
zero
impotent
crapmeister
four oh four
it's not what you know but who you know
unintelligence
disesteem
underestimator
overthinker
cut below
rube
lose one's nerve