What does "a few fries short of a Happy Meal" mean — and why is it funny?

informal, humorous

Meaning

A jokey way of saying that someone is a bit foolish or eccentric.

Where it comes from

A modern American comic insult, a fast-food entry in the long 'short of a...' family of phrases.

Why it is funny

The humor is the small, sad, very specific lack. A Happy Meal is meant to be complete and cheerful, so a few missing fries turn it into a quietly disappointing box — exactly the gap the phrase says is in the person.

Used in a sentence

"He argued with the parrot for an hour — a few fries short of a Happy Meal."