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. It was my wife (who hails from Buckinghamshire) who stared blankly at me when I used it the first time, years ago, and it was only then that I started to wonder how many people knew what it was and who didn't. So, a quick Google tonight and I found a brief reference to it on “A Dictionary of Slang and Colloqual English Slang and its Analogues“:-
A splinter of skin formed near the roots of the finger-nail.
I'm still not sure how widely this is used; whether it's an old expression or just maybe regional.  Any takers?