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.
See also: all, not, there
Farlex Dictionary of Idioms.

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.
See also: all, not, there
Collins COBUILD Idioms Dictionary, 3rd ed.
See also:
  • not all there
  • not for a second
  • not for a minute
  • not for a moment
  • aint
  • ain't
  • not for a/one minute/moment/second/instant
  • not for a instant
  • by no means
  • not by any means