I saw P.O.S. play Nectar last Thursday and wrote about it for the CAB here
"P.O.S. began as a punk rock musician. In high school, [Stefon] Alexander and his friends played punk shows in a band called "Om," in which he first came to be referred to as "Pissed Off Stef." The abbreviation has taken on various and indefinite meanings including, "Product of Society," "Promise of Stress," "Promise of Skill," "Plain Ole Stefon," and "Piece of Shit.""
Definitely not Point of Sales. He's a great performer, go see him if you get the chance.