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.
The indentation in a woman's skin, usually below and between the breasts, made by a tight, underwire bra. This indentation can sometimes resemble a mustache.
I took my girl's bra off and couldn't help but laugh at her brastache!
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 heryoga pants, you can see her bumstache!