1. A person who has a crush on someone but does not show interest, but desperately wants to do so. This usually is the case for the typical nice guy. The word probably derives from the guy who's always in the back who shows interest in a girl but never reveals himself to show interest in her.
1. That guy was totally a backster, I heard from a friend of his friend. He had a major crush on you, he probably didn't want to let you know because you were treating him as a friend.