Alt spelling of new.
On the internet it became a meme when it was attached to a specific genre of music, i.e. nu metal. Nu, in it's more specialized meaning, denotes the re-envisioning of some art (music, film, fiction, etc.) with specific attention to pop-culture references and image as part of the creative process and the whole package.
Korn, Limp Biscuit, System of a Down and Slipnot are all bands that play nu metal.
Quentin Tarantino, Robert Rodriguez, Frank Miller and Takashi Miike are all auteurs of nu in film.