Green’s Dictionary of Slang

1. a rumour, usu. of impending crimes, a tip.

1819		P. Egan Key to the Picture of the Fancy going to a Fight 7: The important ‘whisper’ where the mill is likely to take place [...] However the place is never named as a ‘certainty’.
1849		Era (London) 3 June 3/4: I heerd a whisper from old Tom Taylor, at Epsom, that satisfied me Darkelow warn’t drawn out of the Derby for nothink.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1916		J. Lait ‘Charlie the Wolf’ in Beef, Iron and Wine (1917) 61: The Chief had a bum whisper.
1920		R. Lardner Big Town 167: If they’s ten horses in a race you can walk from one end of the clubhouse to the other and get a whisper on all ten of them.
1939		R. Chandler ‘Pearls Are a Nuisance’ in Spanish Blood (1946) 111: Couple insurance guys tryin’ to deal on a Carondelet Park job [...] You ain’t heard a whisper?
1940		J.G. Brandon Gang War 203: ‘What’s the line, Mac?’ the sergeant asked alertly. ‘The whisper is a running of dope, Tom.’.
1956		R.T. Hopkins Banker Tells All 149: Then came a whisper that Fraser, ‘King of the Jelly Boys’ (gelignite gang), was on a spree in Torquay.
1974		G.F. Newman You Flash Bastard 193: Sneed solicited the help of one of his grasses, Ernie Collier. They met as usual off the manor. ‘You must have heard some whisper about him, he was off your manor?’.
1995		J. Cameron Vinnie Got Blown Away 11: So I grassed a whisper to the SO Saturday morning.

2. a criminal’s lookout man or tipster.

1889		Clarkson & Richardson Police! 320: A watch or watcher ... A marking, stall, whisper, stump, crow.
1971		T. Thackrey Thief 122: This guy, Whisper-man, is one of the best I ever did run across.