The lights are on, but nobody's home saying informal
It is a humorous expression used to say that someone is stupid, unthinking, or inattentive.
What is going on with John today? It’s like the lights are on but nobody’s home.
Hello, Pete? Earth to Pete. He must be daydreaming again—the lights are on, but no one's home.
Don’t try discussing anything intelligent with Alice. The lights are on but nobody’s home, I’m afraid.
Beside this idiom, we can also use "the lights are on but no one's home" with the same meaning.
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This sentence was used since at least the late 1980s. When all the lights are on inside a house, we expect that house to be occupied. It is unusual for a house to be fully-lit when its occupants are out. This expression uses such an occurrence to describe a person who seems fully functional and animated but who is ‘vacant,’ or, in other words, unintelligent or unaware.