What does "a couple of cans short of a six-pack" mean — and why is it funny?

informal, humorous

Meaning

A jokey way of saying that someone is a bit foolish or not all there.

Where it comes from

A modern American comic insult, part of the popular 'short of a...' family that measures a person's wits against an everyday set of things.

Why it is funny

The humor is the cheerful, specific shortfall. The phrase hands you a six-pack with a couple of cans plainly missing, and lets that small, silly gap stand in for whatever the person is missing upstairs.

Used in a sentence

"He keeps his keys in the fridge — a couple of cans short of a six-pack."