Telling an unbelievable lie and getting away with it.
The term derives from "the Boss" who is an anonymous, fictional, public figure. Who lies a lot but always convinces the reciever he is honest.
Mark: "John... Did you really see Haley last night?"
John: "Nah bro, I was doing a 'boss' weren't I"
Used as a way of saying something or someone is stupid, ridiculous, and gay. A form of replacing the word that's gay. When someone says "that's boss" they are referring to something or someone that they do not like
Paul: hey bernard!
Bernard: hey paul
Paul: whatchu doing?
Bernard: doing some Chemistry homework.
Paul: oh, that's boss dude.
Bernard: yea I know, it's so boss.