A shellback is a U.S. Navy term for a sailor who has crossed the equator. Until that ceremony, he is a pollywog.

Ewan MacColl wrote this song about his interviews with the sailor Ben Bright Charles Parker in London. From mudcatter Jim Carroll:

Then somewhere in his seventies, he had been under sail, jumped ship in the US in the 30s and worked with the I.W.W. (Wobblies), given songs to J. M. Carpenter, and had ended up in London acting as an assistant to a escape artist who performed for the tourists visiting The Tower of London. Parker and MacColl recorded him in his bed-sitter in North London until one day they turned up at his home to find he had joined the crew of a cargo vessel heading for Australia. He was last heard of working on a coaster in South Australia. MaColl and Seeger produced a monograph on his life and experiences, and MacColl wrote this based on his description of life at sea.