You know that sweat stain that ladies get under their boobs? The sweat pours down the central boob canal, and out the sides in the underboob regions -
thus forming the sweatstain in the shape of a
mustache...
aka, the Boobstache.
"
yo, check out the handlebar boobstache on the girl on the elliptical."
"
eh, I'm looking at the
porn boobstache on the girl twerking out on the treadmill!"