An RSK, or "Ripped-Shirt Kid" is a person, usually male, who rips and/or cuts the sleeves off their shirts. Most likely a college student who wants to show off their muscles outside the gym. They will wear a ripped t-shirt from a high school football team, often listing the names of their teammates on the back, to the gym and then change into a different ripped shirt when they get home so everyone knows they just worked out. They will almost always wear a draw-string back pack as well. Backwards flat-brim hats are common, and these kids usually travel in pairs with other RSKs. Typically listens to Wiz Khalifa, Kid Cudi, and has been known to defend Justin Bieber.
I saw the boys across the street sitting on the porch and one of them just ripped the sleeves off his buddy's shirt! Such an RSK.