subs. phr. (nautical).A sailor: also OLD SHELL.
1883. Graphic, 12 May, 487, 3. The marine was described as a joey, a jolly, a SHELLBACK.
1884. W. C. RUSSELL, Jacks Courtship, i. It takes a sailor a long time to straighten his spine and get quit of the bold sheer that earns him the name of SHELL-BACK.
1885. J. RUNCIMAN, Skippers and SHELLBACKS [Title].
1901. W. S. WALKER, In the Blood, 29. All excepting the captain, who was a regular quiet old SHELL-BACK.
1902. Athenæum, 8 Feb., 176, 3. Any one of a dozen gaunt and hungry SHELL-BACKS in the forecastle would have supported him.