to be daft, to be really dumb, to be dumber than dumb
To be dumber than a doorknob.
To be about as smart as a rock.
The lights are on, but no one's home.
He's ...
as sharp as a marble
two fries short of a Happy Meal
not the sharpest tool in the shed
not the sharpest/brightest crayon in the box
one bulb short of a Christmas tree
a few cans short of a six-pack
There are many more
here (check out the link I provided in that thread) and
here. We can help you choose our favorites.
