A bro that is always at a party but never seems to get in on drinking games or other party activities. He can be seen casually sipping at his beer whilst chilling from a distance or adding the occasional chuckle and/or witty comment. They are very rarely seen alone, usually in groups of 2 or 3 so as to seem like they are having fun. Can also be abbreviated to BGB.
Yeah guys I'm out this time, I'll just background bro it.
Yo get in on this kegstand, don't be such a background bro!