Green’s Dictionary of Slang

a fool, a gullible person.

1734		C. Johnson Hist. of Highwaymen &c. 68: I like a Cunningham, [...] pretended myself with Child.
1785		Grose Classical Dict. of the Vulgar Tongue n.p: Mr. Cunningham, a punning appellation for a simple fellow.
1796		Grose Classical Dict. of the Vulgar Tongue (3rd edn) n.p.: Cunningham. A punning appellation for a simple fellow.
1811		Lex. Balatronicum [as cit. 1796].
1823		‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 61: Cunningberry, also Cunningham — a half-witted fellow.