A friend who you just happen to call, or get a call from, while you are on the toilet having a bowel movement. The relationship deepens depending on how many times this happens.
Daniel: "Hey Man, what's up?"
Sean: "Oh nothing, just dropping a deuce."
Daniel: "Are you serious? I'm also dropping a loud mound of brown as we speak!"
Sean: "Jeez, dude. That is the second time that this has happened in a week!"
Daniel: "Yeah, buddy. We're Bowel Brothers!"