Northern Irish slang, meaning unfair, cruel/unkind or bigoted/stubborn.
Unfair or Cruel/unkind:
Teacher: Right, everyone is getting double homework this weekend!
Guy 1: That's so fly we don't even deserve it!
Guy 1: Can I've a lend of your pencil?
Guy 2: No way it's mine!
Guy 1: Come on don't be fly!