Cab is short for Cabilarial, an old skateboarder. He named a Switch Frontside 360 on his skateboard his last name, calling it "the Cabilarial". Okay so from there the word mutated into Half-Cab, meaning a Switch Frontside 180. Finally it became a term to denote any Switch Frontside spin, I.E. Cab-3, Cab-5, Cab-7...you get the idea. Snowboarders poached the term from skating, and skiiers in turn poached it from snowboarding. Finally the term (in a skiing context) means to spin from a switch position.