Not Too BrightOne worth keeping
“He don't know come here from sic 'em.”
Can't tell the most basic things apart. Even a dog knows those two.
In the same vein
All Not Too Bright →Dense. All weight, no sharpness.
Looks alert. Isn't.
Not enough sense in there to do the smallest job.
Can't manage even the simplest task.
Not sharp at all.
He's lost whatever sense he had.