not playing with a full deck
1. Not mentally sound; crazy or mentally deranged. A: "Look at that guy talking to himself on the corner." B: "I reckon he's not playing with a full deck."
2. Not very bright or intelligent; 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.
not playing with a full deck
mentally deficient. North American informal A deck in this phrase is a pack of playing cards.