The male organ of copulation and, in mammals, of urinary excretion. The term is commonly used by gym rats/meat heads to surreptitiously discuss their genitalia with others who are so inclined.
A:"Hey man who is that?"
B:"That's my girlfriend. Yea pretty hot eh? As a matter of fact, I just got finished working the Bro Muscle."
B:"It's a slow-twitch muscle that develops best with endurance exercises."
B: "It's located in the lower region of the body and through proper technique renders desirable results."
B: "I just maxed out at 2 reps, which is a personal record! Boo Ya!"
A: Oooo ok. I, I think I know where you're going with this... Does working the Bro Muscle require a spotter? Perhaps a large muscular man like myself? ;)
B: I need an adult!!!!!