Acronym for Controlled Substance Salesperson, which is the politically correct term for one that distributes controlled substances such as marijuana, cocaine, methamphetamines, heroin, or any other substance clandestinely referred to as "that shit".
Dad: I WANT YOU HOME NOW!
Son: omg stfu! oh.... wait, my CSS is calling
Dad: Your wut??
Son: My dealer, fool! NOW STFU! *click*