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.
You see someone who you've been friends with who you havent seen in some time, but you want to invite them out for a drink or dinner and they all of a sudden ignore you, yet they were really chatty with you several months earlier. A clear sign you are a backup friend.
a friend who you use as a backup while your real friend is absent or gone or away at the moment
yo did you see jessica she was hangin with her backup friend lena only while amanda went to the bathroom then she started talking to amanda again what a bitch!!!!