friend of ours
"A friend of ours" is a made guy in the mafia. This term is used when a mobster is introducing another mobster to his mobsterfriends. 
When a mobster is introducing a friend who is NOT a mobster to his mob friends, he will say that the person is "a friend of mine."

This is my friend Nino- he is a friend of ours.

Madonn'! Youz can't touch Vinny- he's a friend of ours!

Bill was not a friend of ours. I don't think we'll be seeing too much of Bill no more.