When a man and a woman are either having sex or just flirting (or anything in between)and the woman assumes a submissive position, most commonly on her knees, then the man jerks-off into the woman's mouth. The woman then swallows his load. Used as a code word in casual conversation and allows for secret requests while in public.
Howard snuck off to give Mary a snackpack at the party.
Allen and Kate laughed when discussing their last snackpack in front of their clueless friends.