Green’s Dictionary of Slang

ben n.¹

(UK Und.) a simpleton, a fool.

c.1698		B.E. Dict. Canting Crew.
1725		New Canting Dict.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict.
1750–1804		B.M. Carew Life and Adventures.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum [as cit. 1785].
1828		Lytton Pelham III 292: Stubble it, you ben, you deserve to cly the jerk for your patter.
1835		G. Kent Modern Flash Dict. 5: Ben or Sam, a raw, a novice.
1848		Flash Dict. in Sinks of London Laid Open.
c.1850		Duncombe New and Improved Flash Dict.
1899		Cornishman 27 July 6/2: Ben, clodpate, cod’s-head, corky-brained [...] are all synonyous, in the language of the canting crew, for fool.
1960		(con. 1950-1960) R.A. Freeman Dict. Inmate Sl. (Walla Walla, WA) 8: Bens – Fools.

In derivatives

benish (adj.) (also bennish)

foolish.

c.1698		B.E. Dict. Canting Crew.
1725		New Canting Dict.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1859		‘Scene in a London Flash-Panny’ Matsell Vocabulum 99: ‘Bell, you’re benish to-night,’ exclaimed Knapp, who probably had a design on the purse.