Green’s Dictionary of Slang

nincom n.

a fool.

1795		Sporting Mag. Jan. V 221/1: Then they call me a nincom.
1807		Irving & Paulding Salmagundi (1860) 310: I give them all up for most absolute nincoms.
1832		T. Hood ‘Ode to Joseph Hume’ Works (1862) II 375: No one but a nincum [...] Would furnish such wide trousers to the Sailors.
1844		T. Hood ‘Masonic Secret’ Works (1862) VII 26: That nincum, Mr. Boggles.
1877		J. Greenwood Dick Temple III 142: You precious pack of nincoms.
1883		J. Greenwood Odd People 101: His behaviour is that of the most consummate nincom, that ever was led with an apron-string [F&H].
1966		D. Niland Pairs and Loners 25: Who do you think you’re putting it over? We’re not nincs, you know. We’re with it all the way.