be not the brightest crayon in the box

To be rather unintelligent, dim-witted, or prone to stupidity. Her new boyfriend isn't the brightest crayon in the box but he's pretty good-looking! I wasn't the brightest crayon in the box when I was in school, but I still managed to find a profession I loved. Yeah, that guy's not the brightest crayon in the box—he keeps pulling on a door that's clearly marked "push."

not the brightest crayon in the box

Not particularly intelligent; dim-witted or prone to stupidity. Her new boyfriend doesn't seem like the brightest crayon in the box, but he's pretty good-looking! I wasn't the brightest crayon in the box when I was in school, but I still managed to find a profession I loved.