Green’s Dictionary of Slang

1. (orig. boxing) a knockout blow; or one that is intended to achieve that aim.

1826		Pierce Egan’s Life in London 12 Mar. 469/1: Dick let fly a nobbler, which told like a cricket ball on Joe’s mug.
1848		Bell’s Life in Sydney 18 Mar. 1/3: Solid [...] let out a tremendous nobbler, which turned Ben half-round.
1848		Flash Dict. in Sinks of London Laid Open.
1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict.
1927		(con. 1835–40) P. Herring Bold Bendigo 147: He tried to land a nobbler right off.

2. by ext. of sense 1, a boxer.

1856

‘A Harrassing Painsworth’ in Yates & Brough (eds) Our Miscellany 8: Tom Doland [...] in high spirits after a slight morning’s work of five-and-forty rounds in the gloves with the Bolton Nobbler.