What planet is (someone) from?

What planet is (someone) from?

That is an absurd or stupid thing to think or do; what is someone thinking? (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 from? Taxes pay for hospitals, roads, our military—literally everything on which our society is built." What planet is he from? He could get someone killed driving that speed along a residential road!
See also: planet, what
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
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