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1.
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.
"Ah look who it is! A friend of mine!"

"I'm going to hook Katy tonight hey."
"Katy? Nah, she's a friend of mine."

"Now now now, Amy is just a friend... of mine."
by mary_mary_quite_contrary November 04, 2011
 
2.
A friend who you went to school with, then transferred, (their your best friend) you transferred with them, and when they got to the new school with you, they hungs out with you. Then they slowly started to turn away from you. Now they're mean to you. And you're no longer their friend.
That girl is a friendofmine.
by Auggie Lopez February 09, 2012