If brains were dynamite he couldn’t blow his nose.
He’s not stupid, he’s just educationally challenged.
I love saying that to my debating buddy in class.
Kitty
The elevator doesn’t quite reach the penthouse does it dear?
Guillotining him would make only an aesthetic difference
about half a bubble off plumb
Needs a starter kit
Loses his socks with his shoes on
Forgets which end of the bus has the driver
Sits everything out
Still waiting for Godot
Forgot to wind his brain stem
He’s a few [nouns] short of a [collective noun].
This person is so dense, light enters the room but can’t get out
He’s half a tomato short of a thick, rich sauce.
[Foghorn Leghorn voice] “Boy’s so dense, I’m surprised there ain’t a black hole around his head.” [/Foghorn Leghorn voice]
I say that about my brother all the time.
How about “You’re stupid!”?
That’s not clever.
How about “One turd short of a B.M.”?
From The Green Mile:
“That boy’s cheese done slid off his cracker”
“He’s a couple slices of bacon short of a B.L.T.”
He has all the survival instincts of pasta.
He’s special. The short school bus kind of special.
He’s as sharp as a brick.
And my favorite, which I use quite often…“That takes a special kind of stupid…”
Not the sharpest knife in the drawer.
He has a pencil sharpener for a barber.
His idea of bright is mood lighting.
Plow through the first couple of dorkoid entries to find some real gems at
this thread.
…dumb as a box of rocks.
…sharp as a kitchen sponge.
And, my personal favorite (since I made it up 
):
…hasn’t got the good sense god gave a cantalope.
MomCat
Thick as Yorkshire fog
Struggles to spell OXO
Breaking a brain in for a moron
Would have difficulty assembling a two-piece jigsaw.
Fell into the well of knowledge and almost died of thirst.
When heads were being given out he thought they said beds - and asked for a big soft bastard.
About half as bright as a brick.
The lights are on, but there’s nobody home.
You’d have to go to school for a year to qualify for wrong.
Only a highly educated person could have an opinion that wrong.
Your thoughts are both interesting and original. Unfortunately, the parts that are interesting are not original, and the parts that are original are not interesting.
A couple of French fries shy of a Happy Meal.
I wish I hadn’t heard all these.
Sorry I didn’t read all the links, but my current fave is:
Couldn’t pour piss out of a boot if the instructions were written on the heel.
[Foghorn Leghorn]
Why if brains was rubber he wouldn’t have enough to make a flip-flop for a one legged budgie.
[/Foghorn Leghorn]
As much good as an ashtray on a motorbike.
As useless as a zip in a dishcloth.
As useless as a chocolate fireguard.
Leaves the MENSA scorers untroubled.
He’d change his mind were not for one small detail.
He believes the advertising.
So stupid he thinks pubic hair is a friend of Bugs Bunny.
Keeps his brain in a shoebox under the bed.