Couldn’t find his ass in a phone booth with both hands, the door closed, and the light on.
Wait, a phone…what?
Dan
If he went to a mind reader they’d charge him half price.
Seconded. 
The elevator only goes down.
The factory’s running, but they’re making something different now.
Every village has one.
Seventeen more of him and we’d have a golf course.
not the brightest bulb on the marquee
Doesn’t have the brains of a newt
When they were passing out brain he got stuck behind the door…
Dumb as a barrel of hair
Dumb as a post
Dumb as a wagon wheel
If brains were leather, he couldn’t saddle a flea.
So stupid if you put his brains in a bumblebee, it’d fly backwards.
In northern Ghana (amongst the Wala people, to be exact) a stupid person “has blank eyes” To make it more intense, you can say he has “blank eyes like the holes in a tree”
Indeed. Did I come over as ever so slightly…tetchy?
Anyway. Here’s a stylistic variant - one where you can substitute pretty much any English county you like; but to honor Mrs Trep*, we’ll go with the well known Buckinghamshire version:
Bucks born and Bucks bred
Strong in the arm and thick in the head.
j
- who grew up surrounded by these people - you see?
Ah well, if we’re touring English counties, we can’t overlook Norfolk, where it is said doctors occasionally marked patients’ records with “NFN” (=Normal For Norfolk) - much of the county being deeply rural with isolated villages and a reputation (however undeserved) for inbreeding.
My stepdad would say, “So dumb he’d pay a dime to see a piss ant eat a bale of hay!”
My brother and I privately agreed that might be worth a dime…
Next stop Yorkshire then. In this context, Queer has the older meaning of peculiar, not quite right. To be said in a confidential tone to a single companion:
All the world’s queer except thee and me. And I’m not too sure about thee.
j
Full disclosure:
I knew this only as a pithy Yorkshire saying, but checking before posting, it turns out it’s a variant of a Robert Owen quote: All the world old is queer save thee and me, and even thou art a little queer. So it seems that some Yorkshire folk are rather smart, and well read to boot.
I like this one from “Calvin and Hobbes”: a low-watt bulb.
“Obtuse enough to be a menace and not stupid enough to be innocuous”*
*description of the British ambassador to Nazi Germany, Sir Nevile Henderson, by British historian L.B. Namier.
Ooh! Too soon!
What? That was sixteen years ago?
Dumber than a coal bucket.
Doesn’t have the brains God gave a fly’s scrotum.
So dumb he thinks Manual Labor is the president of Mexico.
Around here we always added "and leakin’ " to “a quart low”. A quart low and leakin’
“not the shiniest coin in the fountain” which I think I heard on Third Rock from the Sun
Dave Barry talking about his dog (paraphrase):
Her lone thought, rattling around in her brain like a BB in a tuna fish can.
Doesn’t know his ass from a hole in the ground.
Doesn’t know shit from Shineola
As bright as a burned out light bulb.
Talks like a man with a paper ass running through a Forrest fire.
Won’t get out of school unless we burn it down.
Gets red & blue glasses with his report card*
*Because he always has 3 D’s
Any chance you could post the thai characters for this? I’d like to copy them.
For the op:
His dope’s just not that straight
sorry 
Thick and flavorless as a bean sandwich