Green’s Dictionary of Slang

1. a man who runs a game of ‘find-the-lady’ or three-card monte n.; also his accomplice.

1839		H. Brandon Dict. of the Flash or Cant Lang. 164/1: Nobblers – confederates of the thimble-men, who appear to play to induce others to do the same.
1854		G.J. Whyte-Melville General Bounce (1891) 100: There they are, hard at it — nobblers and noblemen, — grooms and gentlemen.
1869		J. Greenwood Seven Curses of London 88: A confederate in the practice of thimble rigging – a nobbler.
1876		C. Hindley Life and Adventures of a Cheap Jack 261: In my young days, there used to travel about in gangs, like men of business, a lot of people called ‘Nobblers,’ who used to work the ‘thimble and pea rig’ and go ‘buzzing,’ that is, picking pockets, assisted by some small boys. [Ibid.] 262: The ringleader of the ‘nobbling crew,’ [...] said. ‘No! we will see you and all your yokels d--- first!’.
1882		Sydney Sl. Dict. (2 edn) 6: Nobblers - Confederates of the thimble men.
1895		C. Crowe Aus. Sl. Dict. [as 1882].
1935		R.T. Hopkins Life and Death at the Old Bailey 63: The following crook’s words and phrases date from the days of the old Old Bailey: [...] confederate of thimblemen – nobbler.

2. one who lames, drugs or otherwise tampers with horses.

1847		‘Epistle from Joe Muggins’s Dog’ in Era (London) 7 Mar. 3/2: I met five nobblers all in a heap together [...] Oh ! ho ! thinks I, you are hereabouts, are you; then there’s suminut up.
1853		G.J. Whyte-Melville Digby Grand (1890) 254: We stepped into Mr Nobbler’s well-filled and well-arranged training stables.
1966		J. Holledge Great Aust. Gamble 65: Of course the stories were packed with horse dopers, nobblers, gamblers and crooked jockeys.

3. a petty criminal operating on or near a racecourse; e.g. a dubious bookmaker.

1840		Eve. Chron. (London) 15 June 4/6: [Bettors] that have suffered by laying more than their stake against favourites, hedging at the last moment with the ‘nobblers’, will compelled to keep within compass.
1849		Paul Pry 16 Apr. 4/1: These fellows are known by the significant term of "Nobblers,'' and are always to be found lounging about the bar and parlours of houses ‘where the odds are taken,’" waiting for strangers, whom they rob, after a fashion that the law cannot grapple with.