Not the sharpest knife in the drawer. 
A few clowns short of a circus. 
A few fries short of a Happy Meal 
A few Cokes short of a six-pack. 
A few peas short of a casserole. 
The wheel's spinning, but the hamster's dead. 
One taco short of a combo. 
A few feathers short of a duck. 
All foam, no beer. (my personal favorite) 
The cheese slid off his cracker. 
Couldn't pour water out of a boot with instructions written on the heel. 
He fell out of the stupid tree and hit every branch on the way down . 
An intellect rivaled only by garden tools. 
As smart as bait. 
Chimney's clogged. 
Doesn't have all his dogs on one leash. 
Forgot to pay his brain bill. 
Her sewing machine's out of thread. 
His belt doesn't go through all the loops. 
No grain in the silo. 
Proof that evolution CAN go in reverse.
In the pinball game of life, his flippers were a little further apart than most.
One fruit loop shy of a full bowl. 
Body by Fisher, brains by Mattel. 
Has an IQ of 2, but it takes 3 to grunt. 
Warning: Objects in mirror are dumber than they appear. 
Elevator doesn't go all the way to the top floor. 
His antenna doesn't pick up all the channels. 
Receiver is off the hook. 
Several nuts short of a full pouch. 
Skylight leaks a little. 
Slinky's kinked. 
Too much yardage between the goal posts. 
Got into the gene pool while the lifeguard wasn't watching. 
A room temperature IQ. 
Got a full 6-pack, but lacks the plastic thingy to hold them together. 
A gross ignoramus -- 144 times worse than an ordinary ignoramus. 
A photographic memory, but the lens cover is glued on. 
A prime candidate for natural de-selection. 
One celled organisms out score him in IQ tests. 
Donated his body to scientists...before he was done using it. 
During evolution his ancestors were in the control group. 
Fell out of the family tree. 
Gates are down, the lights are flashing, but the train isn't coming. 
Has two brains; one is lost and the other is out looking for it. 
He's so dense, light bends around him. 
If brains were taxed, he'd get a rebate. 
If he were any more stupid, he'd have to be watered twice a week. 
If you give him a penny for his thoughts, you get change back. 
If you stand close enough to him, you can hear the ocean. 
It's hard to believe that he beat 100,000 other sperm. 
Some drink from the fountain of knowledge, but he just gargled.