He thinks the Mexican border pays rent.
Doesn’t know his ass from deep center field.
Psst, see post 61. 
An oldie from 60’s high school:
She thinks “Moby Dick” is a venereal disease.
.
If you pronounce the word “gullible” real slow it will sound like “oranges”.
Lightbulb’s burned leftside of his ass and out rightside of his ass.
IQ equals room temperature in the South Pole, outside, at midnight on St John’s Eve*
- (midsummer in the Northern hemisphere, midwinter in the Southern)
Got kicked out of [insert here name of local organization working with mentally challenged folks] on account of being too dumb.
Still on dial-up.
“He’s one of the great minds of the fourth century.”
Thanks to my pal Bill Gately, RIP.
He may talk like an idiot, and look like an idiot, but don’t let that fool you: He really is an idiot.
(Groucho Marx.)
The wheel is turning but the hamster is dead.
Dumber than a bag of hammers
Impaired rectal cranial clearance
Foghorn Leghorn/‘Boy reminds me of Paul Revere’s ride, a little light in the belfry.’/FL
He thinks manual labor is the president of Mexico.
IQ of a Boston Fern.
Hasn’t got the common sense God gave a screwdriver.
Was absent the day they taught graceful.
A few fries short of a Happy Meal.
Splinters in the windmills of his mind.
Cranial rectitis
A wonderfully evocative one I’ve heard only recently:
As useful as a chocolate teapot.
If you set his brain on the edge of a razor blade, it would look like an ant’s egg rolling down the middle of a four-lane highway.
He may be stupid, but at least he’s incompetent.
He’s a fast plant.
I’ve always heard that called a recto-cranial inversion.
And I sometimes have to add a second line dumb as a box of rocks: “a box of stupid rocks.”
Useful as a sack full of assholes with the good ones picked out.