Polliwog

A polliwog is a sailor who has never made an equatorial crossing in his professional capacity. The first time a polliwog makes such a crossing, he is expected (regardless of rank or seniority) to engage in a number of humiliating and disgusting rituals with his fellow polliwogs on the deck of his ship. At the completion of these rituals, he facetiously swears allegiance to King Neptune by kissing a particularly unattractive crewmate playing the role of the ancient Mediterranean deity as part of the light-hearted ceremony, and thus becomes a shellback.