joking

You must be joking! (informal)

something that you say in order to tell someone you do not think something they have said is serious because it is not likely to be true or to happen. 'Did Alex get you a present, then?' 'You must be joking! He didn't even remember it was my birthday.'

See also: joke, must

You're joking! (informal)

something that you say when you are very surprised by what someone has just told you. 'They're getting married, you know' 'You're joking! They've only known each other a month.'

See also: joke, you're