Stupid.

This is one of the many derogatory phrases that emerged in the 1980s which allude to someone 'having a bit missing', 'not all there' etc. The implication being that the victim of the jocular insult lacked a portion of their brain. In this case the deck is of course a deck of cards. Anyone 'not playing with a full deck' wouldn't be expected to make much of a fist of card playing. Other common 'missing' phrases are:

- Doesn't have both oars in the water.
- One sandwich short of a picnic.
- The light's on but no one's at home.
- A few bricks short of a full load.