The act of being a boss. A boss is someone who is simply awesome (at everything, any specific thing, or just awesome in general) and doesn't give one sh*t about it.
Guy: Dude, my younger brother is a boss. He just came up to visit me at college, f*cked my friend with huge tits and left without saying anything to us.
Guy 2: Damn...I don't know if I can really handle that much bossetry.
Guy: Bossetry at it's finest.