be not playing with a full deck

1. To not be mentally sound; to be insane. A: "Look at that guy talking to himself on the corner." B: "I reckon he's not playing with a full deck." He says he's going to start a business selling bees as pets. I think he's not playing with a full deck. Anyone who would go out in a blizzard like this isn't playing with a full deck.

2. To not be very bright or intelligent; to be dimwitted. Jim's a nice guy, but with some of the foolish things he does, I wonder if he's not playing with a full deck. The new manager is nice enough, but he's not playing with a full deck, if you ask me. Tom's one of the top students in the school. His sister, on the other hand—well, let's just say that she's not playing with a full deck.

3. To be acting deceptively (as if one is playing with a deck of cards after having removed some of the cards from it). I doubt that Brian's playing with a full deck—how could he win so much money without cheating? There's no way you're that much better at poker than I am. Are you not playing with a full deck? Hey, people are getting suspicious—they're starting to think you're not playing with a full deck.

be not playing with a full deck

mainly AMERICAN

If someone is not playing with a full deck, they are not being completely honest and therefore have an unfair advantage over other people. This guy is either very good or he's not playing with a full deck. Note: A deck of cards can have cards taken out before a game in order to give one player an advantage.