The act of stealing a T-shirt from those trampy guys who sell dodgy T-shirts outside of gigs. To achieve this objective you politely ask to try it on and then, quick as a flash, run away giggling to oneself. Shouting "booyah!" is optional.
Dude A: "Man, those T-Shirts are shit, but I can't be bothered doing any washing and need a T-Shirt for tomorrow."
Dude B: "Do a T-shirt classic!"
Dude A: "Trantastic."