Green’s Dictionary of Slang

thornback n.

an old maid.

c.1698		B.E. Dict. Canting Crew n.p.: Thornback an old Maid.
1700		N. Ward A Frolic to Horn-Fair 8: She was grown a pretty stale Thorn-back.
1707		J. Dunton Bumography 41: Thorn-Backs (tho’ Maids) as one expresses, Are uniform — in Uglinesses.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1790		‘Bumper Allnight. Esquire’ Honest Fellow 57: They’d giggle and point, would cry out with a yawn, / A thornback has got a red herring with spawn.
1811		Lex. Balatronicum.