Any friendly character in one's life who is something more than a friend: whether it be a person whom one has a caveat over, one's romantic interest, a person one has a history with, a person one hooked up with once, a fuck buddy, a reciprocator of flirting and so on.
“Ok when I introduce you I’m gonna say you’re a friend of mine. That means you’re a connected guy. Now if I says, this is a friend of ours, that would mean you’re a made guy, a capeesh! “ - Al Pacino Donnie Brasco
By Joice
It Happened To A Friend Of Mine
The phrase that people use when saying a ridiculously embarrassing story about something that happened to to them. To save themselves from total humiliation they say "it happened to a friend of mine".
George: God, I am so embarrassed, I can't believe Stacy found out I was cheating with her.
Bill: Don't worry, at least you didn't cheat on her with what you thought was her sister but turned out to be her fat brother.