Too stupid to tell the difference betwenn his ass and a hole in the ground.
In comparison, I sometimes allow as how somebody can tell the difference between his ass and a hole in the ground, but that he does it by taste. As in:
“He can tell his butt from a hole in the ground, but he has to lick it first.”
“If cornflakes were ignorance, he’d be General Mills.”
Hey, if you’re going to steal, steal from the best.
Knitting with only one needle.
One wave short of a shipwreck.
“Half a bubble off plumb.” (Or just “half a bubble off”)
“Doesn’t know his ass from his elbow.”
“A couple sandwiches shy of a picnic.”
“One brick shy of a load.”
“Can’t tell thin sh** from wild honey.”
“Dumber-n-dogsh**”
Not really along the exact same line, but someone once told me that the definition of “gross ignorance” was 144 Aggies. (Now I’m gonna have Texas A&M grads putting out hits on me! LOL) 
He’s so dumb he couldn’t pour piss out of a shoe if the instructions were printed on the sole.
He’s so dumb he flunked “Your Ass And A Hole In The Ground: A Comparative Study”
Your parents don’t call you son because you’re bright.
I’ve always given them the map, the flashlight, and both hands. My other favorite is “If brains were open flames, I’d feel comfortable sending you to the gas station.”
A friend of mine one described herself as “not the brightest tool in the shed”.
QUEEEEEN!!!
I’ve always been fond of “About as sharp as a bag of donuts.”
As dumb as a…
not so bright rock.
retarted primate without thumbs.
Go for simplicity. My standard is “dumber than fungus” or occasionally “dumb as a box of rocks.”
My Dad used to say “If your brains were gas, you wouldnt have enough to power a piss ant’s motorcycle around the rim of a dime.”
He once told me that I was the only person in the world who could fuck up a wet dream. It’s not so much that I’m a stupid person, just inept with mechanical things.
Dumb as a football bat.
The porch light’s on, but nobody’s home.
Not the sharpest knife in the drawer.
He ain’t eating with both chopsticks.
.
.
.
this one describes me well:
I got two brain cells left: one’s a liar and the other’s a fool.
I thought I’d invented one, but it’s got lots of parents. I remember describing Dan Quayle as “Not the brightest bulb in the Senate chandelier.”
If you blow on his ear, you can play his head like an ocarina.
The man’s a violation of the laws of physics: total vacuum from the neck up.
I thought he should blow his brains out, so I gave him a Kleenex.
The only way you could make a thought cross his mind is if you wrote on ammo first.
[hijack]
Incidentally, I was looking at Ken dolls the other day (Christmas shopping for a 3-year-old) and noticed that since Ken was never really anatomically correct (or at least not real graphic ~ thank heavens!) they are now making him with the design of “boxer-briefs” instead of just plain old “buck nekkid”.
[/hijack]
Don’t know shit from Shinola.
Don’t ask that one to catch a bus.
Murphy’s Lawyer
Can’t walk and chew gum at the same time.
uberDave posted:
" Not a ‘dumb as’ but feel free to use this as necessary…
“tighter than a preacher’s cock in a heffers ass”.
Works well when removing nails or unscrewing the lid on the pickle jar. "
I have another one for tightness( either like nails, but better for saying someone is tight with their money )
“Tighter than a nun’s cunt on sunday”
Here are a few expressions that I’ve heard and/or used to describe idiocy:
If he got any denser, light would bend in towards him.
Denser than a block of rebar.
Slower than a squished snail.
Smarter than a speeding bullet!
Has a room-temperature IQ.
Has a full six-pack, but lacks the plastic thingie that holds it together.
Has two neurons playing catch with a single acetlycholine molecule. (Biogeeks might be the only people who use that phrase.)
I used this expression once while I was talking about a particularly moronic person in one of my classes: “He’s probably quite useful as a source of protein, but he certainly shouldn’t be a student in this course.”
And, speaking of idiocy, I should be heading off to my brain-dead statistics class…
That should be acetylcholine! Sheesh!