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 under the influence of drugs.) A: "I think they should just do away with taxes altogether!" B: "What are you on? Taxes pay for hospitals, roads, our military—literally everything on which our society is built." What are you on? You can't drive your car that speed along a residential road, you could get someone killed!