Green’s Dictionary of Slang

1. a cadger.

1873		Sl. Dict. 339: Whisperer a constant borrower.
1900–10		Stephens & O’Brien Materials for a Dict. of Aus. Sl. [unpub. ms.] 166: WHISPERER: [...] generally – a borrower or lug-biter.

2. a racehorse tipster, who expects a share of the supposed winnings.

1900–10		Stephens & O’Brien Materials for a Dict. of Aus. Sl. [unpub. ms.] 53: ‘Whisperers’ tip horses to the backers on condition that they receive a cut out of the winnings.
1903		Sun. Times (Perth) 9 Aug. 4/8: And the welshers and the whisperers, the tic-tacs and the tugs.
1906		Sun. Times (Perth) 18 Feb. 4/7: Jim [...] ’ad turned a turf w’isperer.
1912		Sport (Adelaide) 15 June 6/4: Magistrate Tom Gepp had to deal with a hard case, who was described by the police as a ‘racecourse whisperer’ .
1930		L. Lower Here’s Luck 54: Tip-slingers, urgers and whisperers slunk like jackals through the crowd.

3. (UK und.) a deafulting bookmaker who returns the stake money but can only promises to pay the winnings later.

1935

Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 12: Whisperer: Bookmker who cannot pay out, but who pays back the stake money and promises to send on the rest.