What does "not playing with a full deck" mean — and why is it funny?

informal, humorous

Meaning

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

Where it comes from

A modern American expression. A deck of cards is missing some of its fifty-two, so the person is trying to play the game of life under-equipped.

Why it is funny

The comedy is the unfair card game it imagines. Everyone else is dealt a full deck, and this one player is quietly struggling along with a few cards mysteriously gone — through no fault of their own.

Used in a sentence

"Anyone who tries to iron a shirt while wearing it is not playing with a full deck."