I tell my kids, “It’s a good thing you’re head’s attached”.
And the one I learned from my Russian teacher. He was just explaining the Soviet schooling system, but I’ve kept it.
“Somebody’s gotta plough.”
Oo! Oo! I forgot my true favorite. At work, we have this thing that on any given day you can be either pretty or smart, but not both. Thus when someone screws up, “You’re looking reeaall pretty today!”
Also, my Serbian friend taught me this one:
“This head was not made for two legs.” It’s nicely ambiguous - is the person too smart to be human, or dumb enough to be a dog?
I might have picked it up here – if so, I apologize to the person who coined it for not remembering who you are: Not the swiftest wildebeest on the Serengeti. It’s my default phrase any more.
He don’t have the sense that God gave lettuce. (broccoli … the front lawn …)
She can’t walk and chew gum at the same time.
He is *not *a woodland creature. (more for someone who seems smart but has no practical skills)
Couldn’t get a clue if he rubbed himself all over with clue musk and did the clue mating call naked in a field of horny clues in clue breeding season.
Dumb as a bag of hammers all with “Duh” branded on the handle.
Couldn’t find his ass with an atlas, a compass and a native guide.
Played too much football with his helmet off. (Your brand of the game, obviously. /dry brit)
I think I made this one up.
That guy would make a great door stop.
fine, ‘Couldn’t pour water out of a boot if the instructions were printed on the sole.’
A few clowns short of a circus.
Bumped his head one too many times.
Doesn’t have the sense God gave a carrot.
Doesn’t have the brains God promised an oyster.
“dummer than a hundred chickens”
“he’s about a quart low, isn’t he?”
[Jean Stapleton to Holly Hunter in the movie Michael: “Are you in some way… impaired?”
“You’re so bright I bet your daddy called you boy” (variation on the sarcastic “so bright your daddy called you ‘sun’” pun)
“bit more of a Garfunkel than a Simon”
“His mama handled Propecia”
Whenever I say or do something stupid I beg indulgence for being “a little stupid on my Daddy’s side”.
Apparently originated with LBJ in reference to then-House Minority Leader Gerald Ford (center, Michigan Wolverines, 1931-'35).
::checks scorecard::
Hmmm, we may have a winner here!
This is some funny, funny stuff.
He wouldn’t piss if his pants were on fire.
Forrest Gump wears his ‘I’m with Stupid’ shirt when he goes out with you.
Elvis has left the building.
If brains were gasoline, his wouldn’t be enough to make a piss ant’s go-kart go around the inside of a Cheerio.
He’s as bright as a burnt out lightbulb covered in tar buried in the storm cellar of a coal mine.
Courtesy of my dad.
ETA- Oh, and if you balanced his brain on the edge of a razor, I’d be like bouncing a BB down a 4 lane highway.
Lemme see if the Canonical List of FullDeskisms is still avail…
heh! Sure is!
Thy father rocketh mightily! 
Og be praised - that list has 488 entries!!!
Whoa.
If you put his brain in the belly button of a gnat, it’d rattle around like a BB in a Corn Flakes box.
Nitpick: 1488
He’s a grand short of a list of fulldeckisms.