Imho, "shut up" is a very handy phrase that is not used often enough. Instead, we use much less relevant expressions like "you're kidding, right," or "re-ally??" Or we just smile, silently, gritting teeth beneath. Like when someone is unabashedly boasting about the twenty-six features of their new car, and you're not really interested. When someone is trying too hard to be smart and mouths absolute crap while trying to sound all-knowing. When someone is being overly patronising, or presumptuous and nosy about your life. When someone is needlessly droning on, telling you what they think about the world, or about the shirt you're wearing, and you couldn't, really, care less.
I wish we could do away with the niceities and just say "Shut up".