Green’s Dictionary of Slang

a pickpocket.

1795		Sporting Mag. July VI 204/1: A most daring gang of villains, denominated the genteel knucklers, who [...] supported themselves in extravagance and debauchery by the most atrocious acts of plunder.
1812		Vaux Vocab. of the Flash Lang.
1827		Key of Pierce Egan’s Trip to Ascot Races [printed panorama] He had heard of Cracks, / Spicemen, Knucklers, and Sneaks! / And he longed to be at the head of a Party, To show his authority to lag or to twist.
1833		Morning Courier and N.-Y. Enquirer 31 May 2/2: He is said to be one of the most expert ‘knucklers’ alias pickpockets ever known in this Country, and more than twenty years ago was arrested by Mr. Hays on a charge of picking pockets, for which he was tried and convicted.
1834		(con. 1737–9) W.H. Ainsworth Rookwood (1857) 179: An universal knocking of knuckles by the knucklers was followed by profound silence.
1840		London Mag. Feb. 14/1: ‘There’s gemmen near vot can claw a cly in bang-up style—rig’lar knucklers.
1845		Nat. Police Gaz. (NY) 6 Dec. 121/2: Jim Webb and Bill Thompson, two brilliant ‘knucksmen’.
1848		Ladies’ Repository (N.Y.) Oct. VIII:37 316/2: Knucksman, or Knucker, a pickpocket.
1850		Life in Boston & N.Y. (Boston, MA) 6 Apr. n.p.: The regular ‘cross-man’ [...] is either a ‘knucksman’ (pickpocket) a ‘cracksman’ (burglar) or else [...] a hotel thief, entry thief, etc.
1859, 1860		Hotten Dict. of Modern Sl. etc.
1874		M. Clarke Term of His Natural Life (1897) 58: Patience is a virtue, most noble knuckler!