P.K. ORIGINALLY meant Preacher's Kid. Usually having to do with the irony that a preacher's kid is doing or has commited bad or wrong acts- acts you wouldn't expect from a "preacher's kid!"
Guy 1: Man, your girl, Stephanie is a slut!
Guy 2: You know her dad's a Preacher?
Guy 1: Oh yeah?-you got yourself a PK- huh? They say those preacher's kids are the worst.