Lately I’ve been using “He thinks he’s Susan.” which has the benefit of being completely incoherent outside a select group of people.
A few French Fries short of a Happy Meal.
Heard in the Air Force: “About as sharp as the leading edge of a bowling ball.”
And if you want to insult someone, ask “How long was your Mom pregnant before she gave up Pro Wrestling?”
I’ve heard a couple, let me think
stupidity
“He’s so stupid I’m surprised he can remember to breathe”
Looniness
“A few sandwiches short of a picnic”
“He’s wackier than a bag full of gerbils”
I suppose I’ve heard of others that don’t come right to mind.
for dumb: “as sharp as a bag of wet leather”
Jerry Springer
My brother and I like to invent various expressions like:
“Looks like he got bit by the stupid-dog”
or
“He sure got smacked with the stupid-bat”
Ahh, it cracks us up…
One I heard from Tiny Toons:
“Dumb as a bag of hammers.”
A couple of my own slightly modified for the extremely uncoordinated:
“Couldn’t hit the broad side of a barn from the inside.”
“Couldn’t hit the ocean from the beach.”
“Dumb as a brick.”
“Not the brightest crayon in the box.”
“A few screws short of an erector set.”
Trying to talk to a stupid person: “Impedance mismatch.”
“Dumb as a brick.”
“Not the brightest crayon in the box.”
“A few screws short of an erector set.”
Trying to talk to a stupid person: “Impedance mismatch.”
One I’ve recently been looking for an excuse to use: “A few Hobbits shy of a Fellowship…”
Not the most PC expression but I have always enjoyed: “Yeah, he/she’s pretty special … little yellow school bus special.”
Brain optional with that model?
Stofsky, yours describes me perfectly.
I have to find a way to use ‘too dumb to fuck.’
others:
dumber than stone
room temperature I.Q., and I’m not talking fahreheit.
mind like a foam rubber trap
for the narrow-minded:
mind like a steel trap, rusted shut
from a “Mama’s Family” segment of “The Carol Burnett Show”:
There’s sawdust in the windmills of his mind.
Couldn’t hit the ground with his hat.
When they were passing out brains he thought they said trains and asked for a slow one.
Mad as a cut snake - crazy
Shit for brains - stupid
Carry on like a pork chop - behaving stupidly
F**kwit - idiot
Galah - fool
Slower than a constipated turtle (slow)
To a dumby: “Listen, Gump…”
Slower than a constipated turtle (slow)
To a dumby: “Listen, Gump…”
(Loony) “He/She under-medicated today.”
For the mensa rejects
The lights are on but no-one is home
Fell out the stupid tree and hit all the branches
Intelligently challenged
Needs to borrow a brain cell so he/she has 2 to rub together
Graduated the stupid school with 1st class honors
… and my personal favorite
A Texan 
Stupid:
“You gimpy bastard”
“Not the sharpest tool in the shed”
“S/He’s special…you know…‘special’”