butt must
The odor, often redolent of swamp ass, related to perspiration in the region of the thighs and buttocks; may be caused by improper attire as well as poor personal hygiene, the walk of shame, road marches in Texas, deployments to deserts, or Mormonism.
"Dude, I was on the train the other day and there was this fat chick who must have been the target of a gang bang her butt must was so bad."