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 couldn'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 couldn't!" A: "Oh, believe me, I could."
You couldn't (do that)!and You wouldn't (do that)!
an indication of disbelief that someone might do something. Bill: I'm going to run away from home! Jane: You couldn't! Bill: I get so mad at my brother, I could just strangle him. Tom: You couldn't do that!