The act of one man sexually penetrating another man's ass with his penis. Derived from the film "Brokeback Mountain."
Gee Julian, I sure had a good time with Bruce last night, he was brokeback mountin' me all night long. Pass the herpes cream.
The man in a gay relationship that does the penitrating. One could think of the one partner as the mountain, and the other partner as the mountaneer ready to concure him and plant his meat pole on the summit.
In Fred and Jims gay relationship Fred was the "Brokeback Mountaineer".