When a person(most likely female), attempts to remove unwanted upper-lip facial hair with a lotion or chemical remover(like Nair), and either leaves it on too long, or experiences an allergic reaction.
Today, I wanted to look really good for my date. I was a little self-conscious, so I decided to use Nair
on my upper lip just in case I had a female mustache. Unfortunately, it turns out I'm allergic to Nair. Now I DO have a mustache. A rashstache.
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."