you're joking/you must be joking (also you’ve got to be joking) spokenNO used to tell someone that what they are suggesting is so strange or silly that you cannot believe that they are serious 你(一定)是在开玩笑吧〔用来表示某人的话很奇怪或愚蠢,你无法相信他们是认真的〕 ‘Tell him.’ ‘You must be joking – he’d never believe me.’ “告诉他。” “你一定是在开玩笑吧,他绝不会相信我的。
→ you’re joking/you must be joking at joke2(1)