Foghorn Leghorn (of Chuck Jones/Looney Tunes fame) once remarked:
“Nice boy, but he’s about as sharp as a sack of wet mice.”
what you want is the canonical list of fulldeckisms…currently at 1393
oh shit…did i do that…<ahem> ignore previous post of a twit who didnt read all the previous posts…
<dope slap to head>
That boy’s as useless as a one-legged man at an ass-kicking.
[Foghorn Leghorn]That boy’s about as sharp as a bowling ball.[/Foghorn Leghorn]
Useless as tits on a boar hog. (Pardon me, ladies.)
beagledave-
Don’t be too hard on yourself. Your list seems to have more entries (it was updated yesterday), and not all the entries are the same as the ones on the list Aglarond submitted. We won’t be using any of these to describe you.
Couldn’t pick his nose with instructions.
His IQ rises on a hot day.
Dumber than a box of hair.
Not all his dogs are barking.
Not hitting on all cylinders.
Must’ve been dropped a few times as a kid.
Not the smoothest pebble in the stream, nor the shiniest rock in the bucket.
As thick as a whale omelette.
- six cents short of a nickel
- the elevator doesn’t go to the top floor
- doesn’t check the links to see if he’s repeating someone
(emphasis mine) I think this one speaks for itself.
“All foam, no beer”
“Has two brain cells, and they’re not talking to each other”
“As smart as bait”
The wheel is spinning, but the hamster’s dead.
…is a woman
::runs so fast his shoes melt and the hair blows off his head::
an intellect rivaled only by garden tools.
a coupla clowns short of a circus.
Once, referring to a “friend” who had a habit of f***ing extremely stupid men, I suggested we make a poster for her door:
A picture of a rock, with the copy reading “You must be at least as smart as this rock to ride this ride.”
The first time I heard that was on an old episode of MST3K. It always cracks me up (maybe it’s the thought of Tom Servo saying it). 
Got a couple more:
… has delusions of adequacy.
… is depriving a village somewhere of an idiot.
… should sell when his IQ reaches 50.
. . . dumb as a Bush (or a Shrub)
It’s from Time Bandits. The Evil One is talking to his minion Benson. I don’t know the exact quote, but someone here (SDMB) has it as his/her sig.
Tinker
…courtesy of The Bloodhound Gang
“Not the smartest peanut in the turd.”
Granny on the Beverly Hillbillies: “If brains was bacon fat, that boy wouldn’t grease too big a pan.”
Heard used by a Black govt. worker: “You must be a SPECIAL kind of stupid.”
And, for crazy instead of stupid: “A few bubbles off plumb”
I’ve always used “Not the brightest star in the Milky Way”
- Ahem