  :Supposedly the piercing is named after the porn star Madison, whom was the first person to sport this piercing.  I've never seen any photographic data that backs that theory up, and it may be an urban legend.  I don't know anything about the individual Madison, but given that these piercings have a high rate of rejection, often leaving some really nasty scarring on the front of the neck, if she had one, it might be easy to tell from a decent photo of her neck.  Since the rise of the piercing industry in the 90's, new or experimental piercings tend to be named after the first individual bear them.   