A term used to indicate that someone is "the man." For example, if someone is southpaw, they could kick your ass using only one hand (or "paw), and they most likely engage in numerous sexual encounters (the area down "south") on a regular basis. Most importantly, if someone is southpaw, they are modest about it.
Guiseppi: Dude, you're so southpaw. I wish I could be as cool as you.
southpaw: Nah, I'm not southpaw, although coincidentally, my name is. You, my friend, are the one who is southpaw.