Sometimes you use words and phrases and don't think anything of them...until someone else asks what on earth you are talking about. An example of this is "back-friend". My family have always used this, from my Dad's side I believe. I have heard my grandparents use it in the past and my Dad uses it. Needless to say it's become part of my vocabulary too, but I never though to look at the origin, and always thought everyone knew what it meant.