not be all there
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.
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.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
not be all thereINFORMAL
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.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012