Green’s Dictionary of Slang

the accomplice of a team of card-sharps, the fingerer appears as an old, poor man and dresses accordingly; he then allows himself to be lured into some form of gaming by a group of young confederates, and, through his apparent inability to win, persuades their victim to bet and, inevitably, lose heavily.

c.1561		Awdeley Fraternitye of Vacabondes in Viles & Furnivall (1907) 8: They [...] will appoint one of their Fraternity, which they call a Fyngerer, an olde beaten childe, not onely in such deceites, but also such a one as by his age is painted out with grey heares, wrinkled face, crooked back, and most commonly lame.
1949		I. Shulman Cry Tough! 103: Shake-downers, fingerers, sluggers [...] they too had to obey.