a wild guess

An estimate, prediction, or conjecture based on scant, flimsy, or nonexistent evidence. Sometimes used sarcastically when one believes something is obvious. You get marked down for incorrect answers on this test, so don't just make a wild guess if you don't know the answer. Well, this is just a wild guess but I'm going to say that the answer to the question is Countess Constance Markievicz. A: "Why are you angry at me?" B: "Gee, Brad, why don't you take a wild guess?"