You wouldn't!

You wouldn't!

An exclamation of shock, horror, or incredulity regarding something that someone has threatened or said that they intend to do. A: "I'm sorry, Bill, but we're going to have to repossess the house." B: "No, you wouldn't! My family has been in this house for six generations!" A: "Either withdraw from the election, or I'll share these incriminating photos with the press—and your wife!" B: "You wouldn't!" A: "Oh, believe me, I would."
Farlex Dictionary of Idioms.
See also:
  • You couldn't!
  • (well,) bust my buttons
  • Well, bust my buttons!
  • Well, shut my mouth!
  • (well,) shut my mouth
  • Are you having a laugh?
  • get out!
  • Ye gods and little fishes!
  • Ye gods!
  • (oh,) snap