you must be joking

you must be joking

You must not be serious. What you are saying is very surprising or unreasonable. Twenty bucks for a child's ticket? You must be joking! My painting sold for $5,000? You must be joking.
See also: joke, must
Farlex Dictionary of Idioms.

you must be ˈjoking!

,

you’re ˈjoking!

(informal) used to show that you do not believe something, or that you find something ridiculous: ‘It’s the best movie I’ve ever seen.’ ‘You must be joking!’ ‘The boss wants us to work late again.’ ‘What? You’re joking!’
See also: must
Farlex Partner Idioms Dictionary
See also:
  • you must be joking!
  • you're joking
  • get serious
  • Get serious!
  • serious
  • dead serious
  • aside
  • apart
  • GBG
  • can't/couldn't very well do something