what planet is someone on?

What planet is (someone) on?

What are you thinking? That is an absurd or stupid thing to think or do. (Implies that someone would only do or think that if they were an alien from another planet.) A: "I think they should just do away with taxes altogether!" B: "What planet are you on? Taxes pay for hospitals, roads, our military—literally everything on which our society is built." What planet is he on? He could get someone killed driving that speed along a residential road!
See also: planet, what

what planet is someone on?

BRITISH, INFORMAL
COMMON You can say what planet is someone on? to show that you think someone has crazy ideas or does not know about something that most people know about. What planet are these people on? Do they not read their own headlines? Note: This expression is often varied. For example, you can ask what planet does someone live on? or what planet does someone come from?. He thinks hospitals can make major cuts to their budgets. What planet does he live on? Note: You can also say that someone is on another planet. She was just on another planet, I think. She didn't seem to understand that we are ordinary people.
See also: planet, someone, what