the lights are on but no one is at home

the lights are on, but nobody's home

A humorous expression used to say that someone is stupid, unthinking, or inattentive. Hello, Pete? Earth to Pete. He must be daydreaming again—the lights are on, but no one's home. I kept asking the suspect questions about what happened, and they just kept staring at me blankly. The lights are on, but no one's home, you know what I mean?
See also: but, home, lights
Farlex Dictionary of Idioms.

the lights are on but no one is at home

used to refer sardonically to someone of limited mental capacity.
See also: but, home, lights, no, on, one
Farlex Partner Idioms Dictionary
See also:
  • the lights are on but nobody's home
  • the lights are on, but no one's home
  • the lights are on, but nobody's home
  • for the love of Pete
  • sneaky
  • sneaky Pete
  • for the love of Mike/Pete/God
  • honest to Pete
  • Do we have to go through all that again?
  • find your voice/tongue