1. Definition of being strong, rude and super brave, and referencing the male balls because the harder they are, the manlier the man is.
2. Having your balls masked in steel, or having your balls made of steel.
Louie tried to prove that he had balls of steel by entering the boxing ring without wearing a protective cup, but then ended up losing his ability to reproduce when his opponent violated the standarduniversal boxing rules and uppercut him REALLY hard in the groin. Damn, this guy should be felt sorry for! That shit must've hurt like a living hell!