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.