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.
2009		B. Israel Bachelor Girl 22: f a thornback belonged to no family -- and life spans were so short , many girls at twenty - six were on their own - then she needed to seek out a well-respected, churchgoing male whose wife was schooled in female piety.
2010		T. Kaplan-Maxfield Memoirs of a Shape-Shifter 341: Because I had never married and I lived alone, those who mistrusted me called me ‘Thornback’ — old maid.
2019		Twitter 13 Mar. 🌐 Omg I just found out that spinster used to be reserved for women 23-26 and that after you turned 26 if you were unmarried you became a.... THORNBACK. How fucking great is that name!?
2023		Bluesky 30 Aug. 🌐 I’m just going to have to embrace being a thornback and invest in more top of the line sex toys because y’all cannot be trusted for shit and I’m fucking tired.