(NOUN) A Shooter video game whose main focus is on multiplayer modes and downloadable content, so they typically have weaker single player/story modes.
The term stems from the fact that almost all of your bros play them, and they are shooter games; both Third and First Person. Sometimes, it's the only type of game that they play.
Example: Halo, CoD, Gears
Person 2: No dude, that's totally a Bro Shooter. I'll just wait for Killzone 3