Not the sharpest tool in the box
Food, rug, and a whole colony of ants, short of a picnic
He'd argue with a sign-post
One more brain cell & he'd be a plant.
Stuck on the first rung of the evolutionary ladder
Doesn't have enough sense to come in out of the rain
If brains were dynamite, he wouldn't have enough to blow his nose He's got a room-temperature IQ
Sharp as a sausage
Thicker than a whale omlette
As bright as a five-Watt bulb.