Hi, Jennie. I don't think I've heard the kind of "Shut up!" you mean. Is it an expression of incredulity? My mother-in-law went through a phase where everything anybody said was met with incredulity. A favorite response was "Get out!", which sounds like your "Shut up!"

I was grateful that as an in-law, I could just sit nearby and pretend none of this was happening. After a while I was reading a psychology book, and the author said that incredulity was a fad of the time, and it saved the speaker from having to listen and formulate an appropriate response to the other person. In time, people starting ignoring the MIL, and that broke her of the habit.