It's basically a wild-card word. You can you it for anything. It has both positive and negative connotation.
Also, see: What the Guan?
Origin: a misinterpretation of Sean Kingston's gwan
A: "Dude, did you hear about what Stacy did with that donkey???"
B: "Yeah dude, she is such a Guan"
A: "Dude, I can't believe we got free Doritos!!!"
B: "Yeah, that was some good Guan."