This is a way of sarcastically saying no way?! or really?! or of Course!...but it is completely sarcastic so you dont even have to have a tone with it. The word speaks for itself.
This phrase thrived during the late eighties to early nineties in elementary schools through high schools. It is a way of saying "I already knew that, and am surprised this information is new to you."
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi