Green’s Dictionary of Slang

1. (UK Und.) working as a fraudulent bookmaker, typically taking large bets but putting them down on a less popular horse – if the originally betted horse wins, the bookmaker simply claims the bettor was mistaken.

1936

cited in Partridge DU (1961).

2. a confidence trick that involves the substitution of a fake jewel for a real one.

1989

(con. 1950s–60s) in G. Tremlett Little Legs 10: Working the knocker, totting, or doing the tweedle and the twirl.