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!