Slut is the worst thing a girl can be called. The word slut (or hoe or whore) immediately degrades its victim by labeling her as bad because she is sexual. Sexual slurs have been used throughout history to control female sexuality. Given how terrible the word slut is for women, many people wonder why a girl would ever use it.
Reclaiming the Word: Intent matters. Numerous teens and young women I work with say that they use the term with their friends in an endearing way. Calling each other “sluts” in a friendly manner is an attempt to take back the power of the word. Many oppressed groups have reclaimed the word that hurts them the most. Unfortunately this slur will never truly lose its power until the cultural assumption behind it (i.e. female sexuality is wrong) is dissolved.
Going on the Offensive: Offense is the best defense. All girls exist in an environment where they can be called a slut at any moment. Often, when girls bully a peer in this way it is because they themselves are uncomfortable with their own sexuality. One of the easiest ways a girl can represent herself as a "good girl" is by labeling someone else's sexuality as "bad". It’s just like the classic example of a boy calling another boy "gay" in the locker room because he thinks if he shows disdain for homosexuality, there is no way anyone could possibly think he is gay.