Nobody's playing with a full deck

Meaning

A polite, slightly humorous way to say that someone is not very smart or is acting a little bit crazy. It implies their mental capacity is missing a few cards.

Playful, gossipy

Used in a sentence

• He keeps trying to build a boat in the basement, I think he is not playing with a full deck.
• I love my uncle, but sometimes I think he is not playing with a full deck.
• She told me her neighbor thinks he is a secret agent, so clearly he is not playing with a full deck.