In a movie review long ago critic Gene Shalit once remarked of an actress “It seems that the more beautiful she gets, the dumber she acts. She has never been more beautiful than in [this movie]”. Wonderful backhanded compliment.
“If his I.Q. slips any lower, we’ll have to water him twice a day.” – Molly Ivins, writing about a congressman from Dallas.
A neighbor of my mother’s used to describe one of his kids as “dumber than sled tracks”.
He doesn’t have two neurons to rub together.
There’s a thought (or point) in there somewhere.
Don’t have the brains god gave a turnip (multiple usages)
He thinks everyone has trouble licking self-stick stamps.
Twice failed to become the village idiot through being over-qualified - From a Discworld novel (I think it was Witches Abroad.)
Thick as two short planks.
Most of the others I’ve heard (and can think of) have already been posted.
“That boy couldn’t track an elephant with a bloody nose in a snowstorm”.
If there were a tax on brains, the gov’t would owe him money.
She’s pushing the bounds of ineptitude.
this bitch needs to be hit upside their head with a clue-by-four!
Got two brain cells & they’re moshing
Thicker than a whale omelette. (Blackadder)
I think Dennis Miller used this one: “His ancestors volunteered to be in the control group during evolution.”
For an attractive yet stupid person: “Nice cage. No bird.”
Allegedly said by Lyndon Johnson about Jerry Ford: “That boy played too much football without a helmet.”
And one my cousin once used: “He’s got so few brain cells, he knows them by name.”
! I’ve been reading this whole thread in eager anticipation of posting that… sigh…
oh well, it’s funny anyway 
And when all else fails:
“You’re about one brick short of a brick.”
A couple lesbians short of a Springer episode
He’s so stupid he’d ask women why they don’t like to be called chicks.
The Top 14 New Euphemisms for “Stupid”
[ The Top 5 List http://www.topfive.com ]
[ Copyright 1999 by Chris White ]
14> Routinely outsmarted by cheese
13> Three experts short of an antitrust suit
12> Three-time Darwin Award winner
11> Keeps her brain in mint condition
10> A few planets short of a federation
9> Backstreet Boy in a Talking Heads world
8> Duh! on parade
7> Still cutting with rounded scissors
6> He’s a T-1 line of pure stupid
5> At least one Brady short of a Bunch
4> Sharp as a donut
3> Has a vacancy at the Grey Matter Motel
2> T minus dumb and counting
and Topfive.com’s Number 1 New Euphemism for “Stupid”…
1> “Good afternoon, Boulder Homicide”
And a variant on one already posted…
“He’s not even the sharpest spoon in the drawer.”
I was watching Action last night, and they had a good one.
Actor: I’m as sober as an Ox
Peter Dragon: You were studio schooled weren’t you
Actor: Yep, since the sixth gade
PD: I can tell, you’re smart as a duck
hee hee
I love that show.
Gates are down, the lights are flashing, but the train isn’t coming.
Any slower and he’d be in reverse.
Doesn’t have both oars in the water.
Not the sharpest crayon in the box.
Not the brightest bulb on the tree.
He’s not stupid; he’s possessed by some retarded ghost.
“His train of thought is still boarding at the station.”
I’ve been using that one for years.
As swift as a narcoleptic sloth on Xanax.
Until all the legal manuvering is over, I’m still using “24 Electoral votes short of an election”.