Green’s Dictionary of Slang

a fool, a simpleton.

c.1698		B.E. Dict. Canting Crew n.p.: Simkin a Fool.
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.
1850		‘Ned Buntline’ G’hals of N.Y. 78: For that isn’t me missis’s name at all, ye little slimpskin.
1859		Matsell Vocabulum 80: simkin A fool.
1899		Cornishman 27 July 6/2: Sawny, sap-pate, simkin [...] all synonyous, in the language of the canting crew, for fool.
1944		D. Runyon Runyon à la Carte 127: They beat an old simkin from Iowa for fifty thousand tears of blood.