Green’s Dictionary of Slang

a false, detachable collar, to be worn over a dirty shirt.

c.1698		B.E. Dict. Canting Crew n.p.: Numms c. a Sham, or Collar-Shirt, to hide the t’other when Dirty.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1859		Matsell Vocabulum 60: nums Sham; not real.