Green’s Dictionary of Slang

a police officer, esp. one used to break a strike.

1859, 1860		Hotten Dict. of Modern Sl. etc.
1863		Story of a Lancashire Thief 9: If a single cossack faced him he’d upset the bobby in a jiffy.
1873		Sl. Dict.
1886		Graphic 30 Jan. 23/1: A policeman is also called a ‘cossack’, a ‘Philistine’, and a ‘frog’.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1895		Crowe Aus. Sl. Dict. 20: Cossack, a policeman.
1926		Derby Dly Teleg. 7 Apr. 3/3: Among other slang terms for police-men are rozzers, cossacks, frog; raw lobster, M.P. (member of police) nam.
1931		Framlingham Eve. News 24 Oct. 2: Slang terms which have been applied to the police are ‘cossacks,’ [...] ‘frogs’, ‘blue-bottles,’ and ‘crushers’. Have we not heard constables’ boots described as ‘beetle-crushers’.
1934		S. Walker City Editor 105: [O]ften a Communist who is doing no more than throwing a fit [...] appears in the photograph as the victim of a brutal attack by the cossacks.
1944		R.L. Bellem ‘Killer’s Cure’ Hollywood Detective Mar. 🌐 ‘Oh,’ I sneered. ‘A Cossack, hunh?’.
1980		(ref. to 1920s–30s) Barltrop & Wolveridge Muvver Tongue 56: In the ’twenties and ’thirties when mounted police were used against unemployed demonstrations they were called Cossacks.