Not sure if it was first uttered by Foghorn Leghorn or not, but how about "That boy's about as sharp as a mashed potato sandwich"?

That boys all foam and no beer... :D

I was gonna enter this contest, but, I'm not the shiniest penny in the roll........
:D

...Dain Bramaged..

Talon:cool:

Is them your legs or are you ridin a chicken?

PUNT......

He certainly isn't "The brightest bulb on the string now is he?"

The poor boy, he aint got sense enough to pour piss outa a boot. . . . bless his heart.

Few bricks shy of a load. . .

Ain't playin with a full deck. . . .

GoodOlBoy

"not the sharpest knife in the drawer"
"not the brightest bulb in the chandelier"

You're about as smart as bait.

And lemme tell you, that Varmint Hunter is addictive if a guy's got time to play it!

Lots of gooduns so far, folks. But remember, there are points for originality!

Don't be a niblick shy of a set of clubs, come up with a new one.

"The guy's about 8 clicks shy of being zeroed in."

OK, I have a few of these:

Full throttle, dry tank. or Gasoline engine, diesel fuel.

Lamborghini chassis, moped engine.

Goalie for the dart team.

The space between his ears powers vacuum pumps.

Has six beers, but lacks the little plastic thing to hold them together.

If brains were taxed, he'd get a rebate.

His jogging trail doesn’t go all the way around the lake.

Renewable energy source for hot air balloons.

Knows Atlanta like the back of her hand, but she’s in Chicago.

A few togas short of an orgy.

A couplet short of a sonnet.

Couldn't write dialog for a porno flick.

Enough sawdust between the ears to bed an elephant.

Forgot to pay his brain bill.

Useful as a kick stand on a horse.

Uses his head to keep the rain out of his neck.

Mind like a steel trap: Things wander in and get mangled.

Music by Mozart, choreography by Kramer.

Ok, this one's not original, but I saw it on Google today (on my Google Customized Homepage) under Original Quotes, and couldn't resist:

A husband is like a fire, he goes out when unattended.

(Evan Esar said this... never heard of him though)

A grain shy of minimum.........