NUF is an abbreviation of Novel Update Forum. Novel update is a site that gives links to other sites with English (often illegal/unauthorized) Translations of Asian Novels. If you read this in a chat this might be what the person means.
A slut that should just stop and sit her ass down in a corner and clean her schedule! A nuf nuf usually are the ones that wear short shorts to an ice skating rink and the tightest of dresses to school.