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.
"When I was around 11 or 12 my brother-in-law, who is a physician, gave me the "random fact" that the medical term for breasts is actually BOOBACLES, thus making "boob" the slang term." -ghost-from-tomorrow, Reddit.