Not Too BrightOne worth keeping
“If brains were dynamite, he couldn't blow his nose.”
Not enough sense in there to do the smallest job.
In the same vein
All Not Too Bright →Dense. All weight, no sharpness.
Looks alert. Isn't.
Can't manage even the simplest task.
Not sharp at all.
He's lost whatever sense he had.
Can't tell the most basic things apart. Even a dog knows those two.