An athletic support, or jock strap, holds a jock in place over a male's penis and testicles so as to protect them from direct contact while playing sports. A jock is also commonly referred to as a cup. A SMUCup is a cup for SMU. It is a smaller version than that of an adult or junior size cup.
SMU needs to wear a SMUCup, if he wore a junior size cup to protect his twig and berries, he'd trip over it.