There’s nobody home

there's nobody home

Said of someone whom one thinks is dimwitted or mentally impaired. I've been trying to get information from him for half an hour, but there's nobody home as far as I can tell. She's sweet and very attractive, but there's nobody home when you try to discuss any serious topic.
See also: home, nobody
Farlex Dictionary of Idioms.

There’s nobody home

sent. There are no brains in someone’s head. You twit! There’s nobody home—that’s for sure.
See also: home, nobody
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions
See also:
  • nobody home
  • there's nobody home
  • there's no one home
  • mouth breather
  • mouth-breather
  • partially sighted
  • sighted
  • (one) is not playing with a full deck
  • have nothing between the/(one's) ears
  • I like pie