A play on words combining two terms:
1. Brokeback Mountain, a movie about two homosexual sheep herders who become involved in a relationship that must be kept secret due to societal standards and well as their own safety.
2. Bareback, a term for unprotected anal sex.
It has become insanely played out joke, told usually in offices and other places of work by that unfunny guy who gets all his jokes from mailing lists.
Hurr, Brokeback Mountain? More like BAREBACK Mountain 'cus of all the homo-buttsexing lol!!!111one