be not all there

1. To not be mentally sound. A: "Look at that guy talking to himself on the corner." B: "I reckon he's not all there." My grandfather hasn't been all there for a number of years, which has really had a negative impact on how the kids interact with him. You can't tell people you want to start a business selling bees as pets—they'll think you're 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. My aunt likes to act like she's not all there sometimes, but she's actually incredibly sharp. Dude, are you not all there? You're pulling on a door marked "Push"—that's why it's not opening.

not be all there

INFORMAL

If someone is not all there, they are not very intelligent, or are slightly crazy. I tried to talk to her but she wasn't really all 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