Green’s Dictionary of Slang

usu. in pl., a police officer, the police.

1857		Times 30 Oct. in Ducange (1859) n.p.: ‘On to the slops,’ with this cheer a policeman was attacked by some militia-men.
1866		Wild Boys of London I 146/2: ‘The copper?’ ‘Yes; the slop.’ [Ibid.] 204/1: ‘Cool the slop,’ suddenly shouted Spikey.
1873		Sl. Dict. 296: Slop a policeman. At first back slang, but now modified for general use. [Ibid.] 354: Esclop police, now used to signify a constable only. esclop is pronounced ‘slop’ simply, but the c was never sounded. A policeman is now and then called, by some purist or stickler for etiquette, an ‘esclopnam’.
1874		Manchester Eve. News 16 Oct. 4/2: They [...] might be recognised and watched by the ‘slops’.
1882		Sydney Sl. Dict. (2 edn) 2: Coppers, Cops, Traps, or Slops - Policemen.
1884		Ally Sloper’s Half Holiday 7 June 47/2: To others Samuel Hardstaff is a peeler, a reeler, a copper, a Bobby, a Robert, an unboiled lobster, or a slop, but to cook he is Mr Policeman.
1886		C. Osborne ‘Later On’ 🎵 London’s Police will be made up of men [...] And they wont clout the youngsters who call them a ‘Slop’.
1889		Dead Bird (Sydney) 21 Sept. 6/4: The slops got wind of the affair [i.e. a cock-fight].
1895		Coburg Leader (Vic.) 20 Apr. 4/5: Who were the young lads singing out ‘there they go,’ on Wednesday night when the Slop was after them.
1898		Binstead & Wells Pink ’Un and Pelican 261: If you don’t look slippy you’ll ’ave a slop come along.
1901		W.A. Sun. Times (Perth) 8 Dec. 1/1: The Fremantle p.c. constable has installed his pretty protege in a well-known saloon bar [and] all aspirants for the smile of the siren have to be approved of by the slim slop.
1901		Bateman & LeBrunn [perf. Marie Lloyd] Folkestone for the Day 🎵 I goes up when I sees a grinning ‘slop’.
1906		D. Stewart Vultures of the City in Illus. Police News 15 Dec. 12/4: ‘If the infernal slops (police) want this very clever jagger they must go down the well-hole’.
1906		Gadfly (Adelaide) 28 Mar. 9/2: ‘Drunk an’ disturbin’ the peace, yer worship,’ says the slop [...] I ain’t got no time fer p’licemen, any’ow; but this chap fair took the baker’s shop.
1906		Sun. Times (Perth) 22 Apr. 1/1: A recently-appeared John Slop has already established the walking record [...] he once covered five miles in a day and a half.
1910		Sun. Times (Perth) 31 July 2nd sect. 12/7: ‘There’s a bloke knockin’ ’ell cut of bis ole woman!’ yelled the youth as he knocked up the sergeant [...] ‘If I was you,’ advised the sleepy slop, ‘I’d go and give him a crack on the jor’.
1919		Wodehouse Damsel in Distress (1961) 52: Who knows what horrors might have been, / had there not come upon the scene / old London city’s favourite son, Policeman C. 231. / ‘What means this conduct? Prithee stop!’ / exclaimed that admirable slop.
1926		Eve. Herald (Dublin) 8 Sept. 6/1: Slop is what is known as back-slang. Police spelt backwards is ‘ecilop’ which was shortened into ‘slop’.
1926		Breton & Bevir Adventures of Mrs. May 105: And yet jist when I were doing so much good ’ang me if a slop didn’t come in and says ’e were going to summons me.
1932		M. Marshall Travels of Tramp-Royal 96: That slop’ll pinch me for keeps if you hand me the kick-along.
1936		J. Curtis Gilt Kid 31: A few more have sore heads where the slops have bashed them with their batons.
1938		J. Curtis They Drive by Night 92: The slops must be shaken off by now.
1949		Monteleone Criminal Sl. (rev. edn) 213: slopman A plain clothes officer.
1954		Wodehouse Jeeves and the Feudal Spirit 148: Jeeves had had occasion to bean a policeman with it [...] and the smitten slop had dropped as the gentle rain fom heaven.
1959		I. & P. Opie Lore and Lang. of Schoolchildren (1977) 395: When he has gone round the corner we call him names such as slop, natter knob, or flatfeet.