Green’s Dictionary of Slang

a fool, an idiot.

c.1698		B.E. Dict. Canting Crew n.p.: Nigmenog a very silly Fellow.
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.
1834		‘Jack Downing’ Andrew Jackson 70: Do you take his part you skipe, you nigmenog.
1859		Matsell Vocabulum 59: nimenog A very silly fellow.
1899		Cornishman 27 July 6/2: Nick-nimmy [sic], nigmanog [...] are all synonyous, in the language of the canting crew, for fool.