A sailor who has crossed the Equator. There is a tradition, at least within the US Navy, of tormenting newbies (polliwogs) before admitting them as fellow shellbacks.

Typically, the wogs go through various kinds of hazing, including kissing the belly of Baby Neptune. They then receive a card and/or certificate showing that they are shellbacks and exempt from future initiation rituals. It's considered fairly unwise to lose your card.