It is a reaction where two molecules initiate a backside attack in order to react. Because of its nature, it is used by chemistry students as a joke, implying sodomy or a person that enjoys giving/receiving a "backside attack."
Guy 1:"Hey, are you a SN2 master?"
Guy 2:"Yeah. It's not hard once you get the hang of it."
Guy 1:"I didn't know you roll that way."