The "Bro Shield" is a defensive mechanism possessed only by bros. It is mainly used to ward off negative and or bull shit comments made by non-bros, often referred to as haters because of either their negative or jealous attitudes toward the bro. It is not, however, limited to those types of comments. The power of the Bro Shield can be called upon whenever and wherever a bro sees fit to use it.
Non-Bro: Oh my gosh, that's so lame!
Bro: Psh, Bro Shield bitch.