Bumstache
The visible panty line that is formed on the backside of a pair of fullback (granny) panties by the gusset stitching, going from one butt cheek to the other.
Oh man, every time that Nancy bends over when in her yoga pants, you can see her bumstache!
Bumstache
When one has so much hair on ones anus, it looks like a mustache
Look, Fred has a bumstache