Green’s Dictionary of Slang

1. (UK Und.) a pickpocket’s accomplice.

a.1790		H.T. Potter New Dict. Cant (1795) n.p.: staller an accomplice in picking pockets by keeping up the arms of the person the [sic] mean to rob, to prevent detection.
1809		G. Andrewes Dict. Sl. and Cant.
1821		Flash Dict.
1835		G. Kent Modern Flash Dict.
1845		Nat. Police Gaz. (NY) 11 Oct. 59/2: A number of male pickpockets, [...] have flocked to the Fair at Niblo’s [...] Among the most conspicuous [...] was the notorious ‘Dusty Bob,’ accompanied by three or four ‘stallers’.
1848		Flash Dict. in Sinks of London Laid Open 125: Staller, an accomplice in picking of pockets by holding up the arms of persons.

2. (US Und.) the accomplice to a confidence trickster.

1905

Little Falls Herald (MN) 31 Mar. 3/3: How to Operate the Shell Game with Profit [...] When [the victim] is up against it good and cops the wrong nut, have the staller readyto split him from any hobnails that come up to knock.