To abandon a friend (in most cases, best friend) within their presence in order to hang out or become best friends with another.
It is also used to address to someone who has "flew off".
Guy 1 - "Your flying off on me?!"
Guy 2 - "No it's not what it looks like, we're just talking to each other!"
Guy 1 (a week later) - "Hey there fly off! How's your new friend?"
Guy 2 - "Screw you"