The act of getting chewing gum from your pocket, bag, place of storage, when surrounded by a large (or if you're greedy, a varitably small amount) of people, as so that the surrounding people do not notice you have gum, and hence will not request gum from you, depleting your store that was meant for the next week and a half.
Guy 1: "I see you have a fair amount of Gum there, Guy 2."
Guy 2: "Yeah, I've been Sneak-Gumming all week! Them vultures don't got nothin' on me!"
Everyone: "GOT GUM?! GIMMEGIMME."
Man: "Damnit I've never been very good at Sneak-Gumming".