What does "couldn't hit the broad side of a barn" mean — and why is it funny?

informal, humorous

Meaning

Describes someone with terrible aim, or, more loosely, someone hopelessly inaccurate at anything.

Where it comes from

An American expression from the 1800s. A barn's broad side is one of the largest, easiest targets imaginable, so missing it marks truly dreadful aim.

Why it is funny

The humor is the size of the target being missed. The phrase picks the biggest, most unmissable thing in the countryside and then marvels that someone still cannot manage to hit it.

Used in a sentence

"Don't let him take the penalty — he couldn't hit the broad side of a barn."