you must be joking idiom (also you've got to be joking)
B1 （表示不相信或觉得荒谬）你一定是在开玩笑吧
said in answer to something that someone has said, meaning that you do not believe they need said it seriously, or you think it is a stupid thing to say
You've got to be joking if you think I'm going to stand in the rain watching you play tennis! 你以为我会站在雨中看你打网球，你准是在开玩笑吧！