Not the brightest crayon in the box.
A few cards short of a full deck.
As smart as a box of hammers.
A pork chop short of a mixed grill.
A couple of 'Roos short in the upper paddock
Couldn’t win a debate with a cabbage
Not the swiftest wildebeest on the Serengeti.
Not the brightest bulb in the chandelier.
Dumb as a bag of spatulas.
A couple fries short of a Happy Meal.
Not hitting on all eight.
Dumb as a sack of rusty door knobs.
Number than a pounded thumb.
smart like streetcar
A few sandwiches short of a picnic.
Dumb as a bag of hair.
Only one oar in the water (from Johnny Carson, I think)
Sharp as a marble.
Dumber than a box of rocks.
Elevator doesn’t quite reach the top.
Not the brightest bulb on the tree.
Not firing on all cylinders.
Couple straws short of a bale.
Takes an hour and a half to watch “60 Minutes”
Sharp as a bowling ball
Dumber 'n sled tracks
A sandwich short of a picnic
As thick as two short planks