Green’s Dictionary of Slang

a fool, ‘a meer Cod’s head’ (B.E.).

c.1698		B.E. Dict. Canting Crew n.p.: Nick-ninny, an empty Fellow, a meer Cod’s Head.
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.
1899		Cornishman 27 July 6/2: Nick-nimmy [sic], nigmanog [...] are all synonyous, in the language of the canting crew, for fool.