Someone who is really nice and thoughtful one day(like Obama), and tries to kill you the next(like Bin Ladin).
You: "hey, could you help me with my english homework?
Friend: "sure thing, bro!"
You: "Thanks for helping me with my english the other day, but could you help me with my math now?"
Friend: "no, but I can give you a map of how to get out of here before my M-16 meets your face!!