Sharp as a Marble
If you can't figure out what this means, it's probably you.
Person 1: He's not the sharpest tool in the shed.
Person 2: Not the sharpest? He's as sharp as a marble.
Person 2: Not the sharpest? He's as sharp as a marble.
Sharp as Marbles
A way of describing someone as ignorant. Like saying one is not the brightest crayon in the box; One taco short of a Grande Meal; Not the brightest bulb in the socket; Dumber than a box of hair; etc...
What the fuck was I thinkin tellin the cop he was a fat pig AlphaDouche?? Man, I'm as sharp as marbles...