He's playing with half a deck

Meaning

This is a polite way to say someone is not very bright or lacks common sense. It suggests they are missing some of the necessary equipment to complete a full set of logic.

Dismissive, folksy

Used in a sentence

• I would not trust Dave with the tax returns because he is playing with half a deck.
• The poor kid tried to install the shelf without using a level, clearly playing with half a deck.
• She suggested we paint the floor with house enamel, and honestly, she is playing with half a deck.