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