What does "not the brightest crayon in the box" mean — and why is it funny?

informal, humorous

Meaning

A gentle, jokey way of saying that someone is not very intelligent.

Where it comes from

A modern comic insult, a close cousin of 'not the sharpest tool in the shed', swapping the workshop for a child's box of crayons.

Why it is funny

The humor is the soft, almost sweet way it lands the insult. It puts the person in a colourful box of crayons and notes they are simply not the bright one — a putdown delivered in the language of a kindergarten art class.

Used in a sentence

"She's lovely company, just not the brightest crayon in the box."