Not the sharpest crayon in the box.
Drank too much from the stupid tree.
How much stupid did you drink this morning?
How do you do that?.... Turn all you're smarts off?
Rocket surgeon.
Management material.
Superblond.
Sly like a rock.
Not the brightest brick in the box.
A real box of bricks.
One brick shy of a brick.
Can't walk, talk, and chew gum at the same time. - OK, I never say that one, but I did watch a supervisor choke on his gum once because he was trying to walk and talk too. No joke.