When a woman is breast feeding her baby, her nipple grow to approximately 3 inchs. A Nipple Whip is when you whip someone in the face with your extended nipple.
Bob: "Yo are you done breast feeding that baby so we can get it on?
Catherine: No! That was rude! I hate you. Take this!
Bob: Ow, that Nipple-Whip hurt like a bitch!