A romantic relationship that is like a Sharpie: it starts out fine and then it dries out and dies, like a marker would during use. Normally, these relationships do not last long, but some can go on for years until they go flat and stop.
Heartbroken Girl: Why did he leave me? We had so much for each other! I'll never be loved again! *girl sobs deeply*
Friend: Don't worry, kid. It was a total Sharpie relationship. He was cheating on you with another gal. It would have died out anyway like most flings do.