What does "knee-high to a grasshopper" mean — and why is it funny?

informal, humorous

Meaning

Very small or very young; often used of someone remembered from early childhood.

Where it comes from

An American comic exaggeration from the 1800s, one of several 'knee-high to a...' phrases that competed before the grasshopper won out.

Why it is funny

It measures a child against an insect — and then only reaches its knee. The image is impossibly tiny, and grasshoppers have no knees you would ever notice, which makes the unit of measurement as funny as the result.

Used in a sentence

"I've known him since he was knee-high to a grasshopper."