Green’s Dictionary of Slang

a fool, a simpleton.

c.1698		B.E. Dict. Canting Crew n.p.: Nikin, a natural, or a very soft creature.
1725		New Canting Dict. [as cit. c.1698].
1727		L. Welsted Dissembled Wanton II i: The Wits and Poets, make it their Business in their Plays and Prologues to abuse their Betters [...] giving them Nick-names, such as Nikin, Gripe [etc.] .
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
1743		H. Mann in Walpole Corr. 9 July N.S. (1955) XVIII 264: Poor Nikin her husband answered a person who asked him if he had any children, ‘Oh no, Sir; if I had I should not be here.’ .
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Nickin, nikey or nizey, a soft simple fellow.
1811		Lex. Balatronicum [as cit. 1785].
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue [as cit. 1785].