An ugly prank played after camping with one's best friend or girlfriend and eating a generous amount of grilled hot dogs or Brats. The pair hike to a nearby summit (preferably one of 8000' or more). Once at the summit and both persons are ready to expel the winers, the prankster takes initiative to find a viewable open location to do the deed. The prankster drops his/her pants whilst yelling to the prankee to look towards their bare ass, which should then be blowing mud toward the prankee as they look over. Hence the Filthy Mountaineer.
We hiked and hiked. Suddenly, I really had to go shit so I dropped my pants in front of Cindy and gave her a Filthy mountaineer!