used to show insignificance in one's speech or state of mind. It is in reference to one who is mentally handicapped. If some one says something unintelligent or replys to a question or answer with something unintelligent, the other person responds with "Neh Neh!"
Kid1: Im gonna go outside and shoot some ball
Kid2: hey. you know that its raining outside. that means you could get wet.
Kid1: Neh Neh!