be not all/quite there

be not all there

1. To not be mentally sound; to be crazy or mentally deranged. A: "Look at that guy talking to himself on the corner." B: "I reckon he's not all there."
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 think he's not all there.
See also: all, not, there

be not all/quite ˈthere

(informal) think slowly because of low intelligence, illness, drugs, etc: Are you sure he’s all there? OPPOSITE: all there
See also: all, not, quite, there