Green’s Dictionary of Slang

whistler n.1

1. a broken-down horse, whose breath whistles in his lungs.

[UK]J.C. Apperley Nimrod’s Hunting Tour (1874) 149: This mare was sound when I turned her out, but she came up a whistler.
[UK]‘Epistle from Joe Muggins’s Dog’ in Era (London) 2 June 6/3: Mildew [is] a andsome oss, and agood ’un too, or I never seed one, if he arn’t a bit of a wistler, witch I suspekts. If so his good looks arn’t of no use.
[UK]Farmer & Henley Sl. and Its Analogues.

2. (UK Und.) a counterfeit halfpenny or farthing [the false ring when tapped].

[Aus]Vaux Vocab. of the Flash Lang.

3. the proprietor of an unlicensed spirit-shop in a prison [his drinks are kept hidden so that when the police raid they can whistle for them].

[UK]W.T. Moncrieff Tom and Jerry III v: The haberdasher is the whistler, otherwise the spirit-merchant, Jerry.
[UK]Dickens Pickwick Papers (1999) 609: The turnkey knows beforehand, and gives word to the whistlers, and you may whistle for it when you go look.
[UK]R. Nicholson Rogue’s Progress (1966) 118: All spirits being contraband in prison, the profession of the ‘whistler’ (for such is the vendor of ‘ardents’ denominated) was once very profitable.

4. a bullet [abbr. blue whistler under blue adj.1 ].

[US]Spirit of the Times (N.Y.) 17 Oct. 98/2: Walking back to my stand, I sounded the ‘buck-load’ of sixteen whistlers in my right-hand barrel [DA].

5. a revolver.

[UK]J. Manchon Le Slang.

6. anything especially large.

[UK]J. Manchon Le Slang.

7. (US Und.) a police car.

[US]J.L. Kuethe ‘Prison Parlance’ in AS IX:1 28: whistler. A police car.
[US]Monteleone Criminal Sl. (rev. edn).
[US]Goldin et al. DAUL 236/2: Whistler. (South) A police car. (More common in large cities: prowl car.).

8. a female railway porter.

[US]Weekly Tel. 28 Feb. n.p.: The war has brought into being many nicknames [...] women railway porters are known as ‘whistlers’ [etc.].

9. (US Und.) an informer.

[US]N.Y. Times 15 Dec. SM16: Singer: an informer [...] also whistler.