"You're so x" can be used in the sense that "x" represents any word you want it to be. In the same sense that in a mathematical equation, "x" would represent whatever number and you would try to solve it. Here, the "x" represents a word, and well, you're gonna have to rely on a person's tone of voice or whatever to figure out what kind of word they're using.
You could even add more equation-like thingies. x+y²
This is so dorky : Whatever. It lets people think about maths..