Ways to say someone is stupid:

'round the bend
52 cards short of a deck, and playing with the jokers.
A few beers short of a six-pack.
A few clowns short of a circus.
A few crumbs short of a crouton.
A few fries short of a happy meal.
All foam, no beer.
All the marbles aren't round.
Am I getting smart with you? How would you know?
An experiment in Artificial Stupidity.
An intellect rivaled only by garden tools.
Antenna doesn't pick up all the channels.
Asks to have his/her pizza sliced in 4 pieces because he/she can't eat 8.
Belt doesn't go through all the loops.
Body by Fisher, brains by Mattel.
Book is full of blank pages.
Brilliant as a black-hole in space.
Brilliant as a burnt-out light bulb.
Bubble off of plumb.
Cheese slid off of his cracker.
Chimney's clogged.
Christmas tree doesn't have an angel on the top.
Cornbread didn't get baked.
Couldn't find his/her way out of a wet paper bag.
Couldn't pour water out of a boot with instructions on the heel.
Dealing with a full deck of jokers.
Dealing with a half a deck.
Dip stick doesn't quite reach the oil.
Doesn't have all his cornflakes in one box.
Doesn't have all his dogs on one leash.
Doesn't have both oars in the water.
Doesn't know much, but leads the league in nostril hair.
Driving with the engine shut off.
Dumb as fertilizer, but not as useful
Dumber than a box of hair.
Dumber than a box of rocks.
During evolution his ancestors were in the control group.
Elevator doesn't reach the top floor.
Fell out of the stupid tree and hit every branch on the way down.
Florida voter.
Forgot to pay the brain bill.
from the shallow end of the gene pool.
Gates are down, the lights are flashing, but the train isn't coming.
Gone fishing.
Half bubble off.
Has an IQ of 2, but it takes 3 to grunt.
Has reached the bottom of the heap and is now digging.
Having a blond moment.
He/she is depriving a village of an idiot.
He/she isn't stupid, he/she's ignorantly endowed.
High priority recipient when they start brain transplants.
His brain isn't in the head on top.
His men would follow him anywhere - just out of curiosity.
I feel bad about having a battle of the wits with an unarmed person.
If brains were taxed, he'd get a rebate.
If he/she had another brain, it would be lonely.
If you want to have a battle of wits, I'll take out my brain too and we can start even.
In the pinball game of life, his/her flippers are a little too far apart.
Is out of his depth in a car-park puddle.
Is overdrawn at the memory bank.
Isn't paid to be smart, and so far they got their money's worth.
Isn't the brightest bulb on the Christmas tree.
Intelligence of a Carrot.
Learned to count in Florida.
Lights are on, but nobody is home.
Makes a house brick look clever.
Mind, is slower than molasses on on winter's day.
Missing a few buttons on his remote control.
Must be twins - no one person can be that dumb/stupid.
Nice kid, but sharp as a grapefruit.
Not cooking with all four burners.
Not got both chopsticks in the rice.
Not playing with a full deck.
Not the brightest crayon in the box.
Not the sharpest knife in the drawer.
Not wired to code.
On a windy day you can see the leaves blow through his head.
One brick short of a load.
One crayon short of a box of 89.
Photographic memory, but the lens cover is on.
Proof that evolution can go in reverse.
Receiver is off the hook.
Sewing machine's out of thread.
Slinky's kinked.
Stayed on the Tilt-A-Whirl a bit too long.
Strong like a bull ..... smart like a tractor.
Surfing in Nebraska.
Smart as bait.
Some drink from the fountain of knowledge, but he just gargled.
Standing close to her, you can hear the ocean.
Too much yardage between the goal posts.
The butter has slipped off his pancake.
The cheese slid off his cracker a long time ago.
The interior decorator is on ,1) strike 2) vacation 3) Maternity leave to find a replacement.
The wheel's spinning, but the hamster's dead.
Thinks Santa Monica comes once a year.
Three beers short of a six-pack.
There's toys in the attic, but there's no grain in the silo.
When he/she turned 40 his/her age and his/her IQ matched.
Would lose to fishing bait in a thinkin' contest.

More...

I have a supervisor that we call "old Etch-a-sketch head," One good shake and everything's gone.

We like to say that when his wife blows in his ear, he says "Thanks for topping me off."

His elevator doesn't quite make it to the top floor. In fact, it hardly gets out of the basement.

He couldn't organize a nose blowing. He'd have a hand full of snot, before he realized that paperwork is part of the job.

When people inquire about how I get along with my boss, I tell them that we are just like the Himer twins. I am Wisen Himer and he is Altz Himer.

If his/her brains were dynamite, he/she couldn't blow his/her nose.