Taken from the movie D2: The Mighty Ducks and the characters of Reed and Portman. This can be used to refer to any two guys who are really close friends. They share the same interests and usually partake in them together.
Occasionally, there can be a third member who sometimes joins the bash brothers. This person can be referred to as the third bash brother, or "Wu-Wu-Kenny Wu."
Jeff: "Where are Neil and Cory?"
Bryan: "They told me they were going to their club meeting and then to their intramural basketball game after that."
Jeff: "Man, those two do everything together. They really are bash brothers."
Bryan: "Yeah. But you hang out with them a lot, too. You are like the third bash brother, Kenny Wu."