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.