This one is my personal favorite. I used to have it on my screen saver, at work. My boss looked at me funny the first time she saw it. Why? She didn’t get it. Bright lady in most respects, but sometimes the simple things just sail right past her.
You could practically hear it flying past her:
WHOOOOOOOSH!!!
Lord knows I’ve had that feeling. Thank god I’m good at nodding and looking thoughtful and laughing anyway and other assorted “I Got It!” tactics.
Couldn’t find his asshole with a funnel.
Doesn’t have both oars in the water – can’t even find the damn boat.
Knitting with only one needle.
Encyclopedia’s missing some volumes.
His drill doesn’t have a full set of bits.
Easier to count the bricks left than the bricks missing.
Not only a few brick short, but is missing someone else’s bricks as well.
Mind like a steel trap - anything entering gets crushed and mangled.
Suffers from Clue Deficit Disorder.
Doesn’t just know nothing; doesn’t even suspect much.
During evolution his ancestors were in the control group.
Marching to a different kettle of fish.
On permanent leave of absence from his senses.
People around him are at risk of second hand idiocy.
Mind readers charge him half price.
Has a mind like a steel sieve.
The “official” term is “fulldeckisms”…there is a somewhat comprehensive list (over 1300 for your viewing pleasure) here
One I say about my brother:
Boy’s so dense, I’m surprised there’s not a black hole around his head.
He’s got two brain cells left, and they’re fighting.
A few dimes short of a roll.
Doesn’t know his ass from a hole in the ground.
Slower than a herd of turtles stampeding through peanut butter.
As sharp as a pound of wet liver.
My dad always likes to call me “bongwater for brains”.
Who told you?
5 RAM in a 25 RAM world.
If you put his brain on the edge of a razor blade, it would look like a BB rolling down the middle of an 4 lane highway.
You did, just now. 
He’s not the sharpest knife in the deck!
That’s the first thing I thought of when I saw this thread. I went and popped it in the CD player and then I got to you comment, so since I have the lyric out, here they are.
When the outside temperature rises
And the meaning is oh so clear
One thousand and one yellow daffodils
Begin to dance in front of you-oh dear
Are they trying to tell you something?
You’re missing that one final screw
You’re simply not in the pink my dear
To be honest you haven’t got a clue
I’m going slightly mad
I’m going slightly mad
It finally happened-happened
It finally happened-ooh oh
It finally happened-I’m slightly mad
Oh dear!
I’m one card short of a full deck
I’m not quite the shilling
One wave short of a shipwreck
I’m not my usual top billing
I’m coming down with a fever
I’m really out to sea
This kettle is boiling over
I think I’m a banana tree
Oh dear, I’m going slightly mad
I’m going slightly mad
It finally happened, happened
It finally happened uh huh
It finally happened I’m slightly mad-oh dear!
I’m knitting with only one needle
Unravelling fast its true
I’m driving only three wheels these days
But my dear how about you?
I’m going slightly mad
I’m going slightly mad
It finally happened
It finally happened oh yes
It finally happened
I’m slightly mad!
Just very slightly mad!
And there you have it!
Ya know, these descriptions are also used to call someone insane.