A rash or red area on the upper lip that appears similar to that of a mustache. Sometimes caused by dry-shaving, waxing, or some type of allergic response to something.
Guy 1: "Becky was looking so hot today, but she really shouldn't have gotten her upper lip waxed. It left a massive rash stache."
Guy 2: "Yeah, but I'd still ride that ass like a monkey on a tricycle."