What is a "Shellback" in Navy terminology?

A "Shellback" refers to a sailor who has successfully crossed the equator for the first time. This term holds significant cultural importance within naval tradition, as crossing the equator is often marked by a ceremonial initiation known as the "Line Crossing Ceremony." During this ceremony, sailors undergo various rituals and challenges, symbolizing their transition from a "pollywog," which denotes a novice who has never crossed the equator, to a "Shellback," indicating that they are now part of the exclusive group of sailors who have completed this rite of passage. This concept highlights the camaraderie and shared experiences within naval service, creating a bond among those who have undergone the crossing.

The other answers do not accurately capture the traditional meaning of "Shellback" and refer to different aspects of naval life or ranks, providing a contrasting context to the specific significance of the equatorial crossing.