Not the sharpest tool in the shed idiom informal
This idiom refers to sharpness which means mentally quick, so not the sharpest means not intelligent.
My girlfriend is not the sharpest tool in the shed when it comes to motorbikes.
He is not the sharpest tool at work, but he is doing alright.
To have no money
Used to describe someone who is naive, gullible, inexperienced, easily fooled, ignorant, unsophisticated, etc.
To say, or believe something or someone that seems completely crazy, delusional, or stupid
A person who is low-witted or stupid
To be extremely stupid; not to be alert
The exact origin of this expression is unknown, but it's first found in the Colorado Springs Gazette Telegraph newspaper, July 1994:
Critics said that Cliburn was an intuitive artist, and that once his intuition was exhausted, he had little else to say about a piece — which was a fancy way of saying that Cliburn wasn’t the sharpest tool in the shed.