1. Used alone in response from a request from the person you are talking to to repeat something you have said. You mean that you won't bother because what you said before was not very important. English = "It's nothing!", "Forget it!", and so on.
2. Used alone to deny that you have done or said something. English = "I haven't done a thing!", "I've done nothing!", "I didn't say anything at all!", and so on.