"The sailor's commendatory term for the landsman's old salt. Some authorities say that it comes from his back being bent like a shell; but it seems more probable that the implication is that the shellback is growing barnacles from having been at sea so long.The term is fairly well known to landspeople. The degree of "able shellback," signed by Rex Neptune, is currently  being conferred upon men crossing the Line for the first time aboard troopships."
Sea Language Comes Ashore, Joanna Carver Colcord. Cornell Maritime Press, N.Y. 1945.