Green’s Dictionary of Slang

a shirt collar, a fake shirt.

c.1698		B.E. Dict. Canting Crew n.p.: Jacob [...] sham or Collar Shirts.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
c.1750–1882		B.M. Carew Life and Adventures.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1809		G. Andrewes Dict. Sl. and Cant.
1811		Lex. Balatronicum.
1821		Flash Dict.
1835		G. Kent Modern Flash Dict.
1848		Flash Dict. in Sinks of London Laid Open.
1859		Matsell Vocabulum.
1881		Trumble Sl. Dict. (1890).