A person who is someone you know and are "friends" with, but isnt treated like a friend. You are treated like an acquaintence. This is a person one only talks to when their actual friends are away. The person who is the back-up friend is treated poorly and is usually someone who doesnt have a lot of friends or gets a lot of dates. The backup friend is someone who gets the short end of the stick when it comes to friendship. The person who sees you as a backup friend doesnt care about you and isnt concerned with any part of you as long as you are someone they can come to when they want to converse with someone when their real friends have disappeared.