Green’s Dictionary of Slang

1. (also fly, fly bull, fly copper, fly dick, fly sleuth) a plainclothes police officer, a detective.

1857		Nat. Police Gaz. (NY) 13:33 11 Apr. 3/1: Some of these fly coppers in piping a place off should keep their ogles out of the sun.
1865		Leaves from Diary of Celebrated Burglar 5/2: We had not ‘worked’ it long before the ‘fly-cops’ were out in quest of us [...] but we were as ‘fly’ as they, if not more so.
1867		Nat. Police Gaz. (NY) 7 Sept. n.p.: Five ‘fly coppers’ who had more of the ‘queer’ bonds in this case than had Dr Shine.
1872		G.P. Burnham Memoirs of the US Secret Service 98: The ‘fly-cop’ who had this koniacker now in hand understood his biz’ and he ‘chaffed’ him right pleasantly.
1874		Derbyshire Courier 12 Dec. 7/1: Local Flash language [...] A fly, or blue-bottle, a policeman.
1881		City Argus 2 July 4/4: ‘Jimmy,’ as he is familiarly called by the ‘fly cop,’ attempted to get into Banker Sather’s cash-box—was caught ‘dead to rights,’ and now languishes in the city Bastile [DA].
1893		J. Hawthorne Confessions of Convict 19: He had sojourned for four years for ‘glimming at bank-businesss,’ as the offence is designated by crookology and by municipal-bacteria-fly-cops.
1895		J. London ‘’Frisco Kid’s Story’ in High School Aegis X (15 Feb.) 2–4: I took yer fer a fly cop.
1895		Crowe Aus. Sl. Dict. 30: Fly Cop, a sharp officer.
1897		J. London ‘The Road’ in Hendricks & Shepherd Jack London Reports (1970) 311–21: Attempt to translate this : – Hit a fly on the main-drag for a light piece [...] On the main street I begged a policeman in citizen’s clothes for a small sum.
1899–1900		C.L. Cullen Tales of the Ex-Tanks 165: I just seen a fly sleut’ from Pittsburg pass along.
1900		Flynt & Walton Powers That Prey 32: A town as big as this can find its own guns without callin’ in private fly cops.
1903		Nebraska State Jrnl (Lincoln, NE) 14 June 9/6: [T]wo of the best flies for one of the well-known detective agenies.
1908		B. Fisher A. Mutt in Blackbeard Compilation (1977) 32: If I could only win enough to bribe these fly cops, I’d be all right.
1908		‘O. Henry’ ‘The Shocks of Doom’ in Voice of the City (1915) 98: I didn’t know but what he was a fly cop.
1912		A. Berkman Prison Memoirs of an Anarchist (1926) 127: The Captain of the night watch is ‘fierce an’ an ex-fly’.
1912		Day Book (Chicago) 23 Mar. 15/2: Next day when Carry starts for school her pa and a fly cop is trailing her.
1915		Goodwin’s Wkly (Salt Lake City, UT) 16 Oct. 6/1: ‘Murder! don’t do that,’ said the fly-dick.
1917		F. Packard Adventures of Jimmie Dale (1918) I ii: Wot’s de fly cops doin’ out dere?
1918		J. McCulley ‘Thubway Tham’s Thanksgiving Dinner’ in Detective Story 26 Nov. 🌐 “It ain’t right for a man to be pethtered all the time by the thame fly cop!
1924		G. Henderson Keys to Crookdom 402: Detective [...] bull, fly cop.
1925		G.H. Mullin Adventures of a Scholar Tramp 218: We knew he was a fly-cop [...] ‘Put up yer mitts!’ snapped the bull.
1927		E. Wallace Squeaker (1950) 37: Barrabal, eh? [...] That’s the fly – the detective, who is getting himself talked about just now?
1930		‘Dean Stiff’ Milk and Honey Route 203: Dick or fly cop – A detective either in public or private employ.
1931		Irwin Amer. Tramp and Und. Sl. 78: Fly Dick.–See ‘fly ball,’ ‘fly cop.’.
1933		H. Craigie ‘Reverse English’ Detective Nov. 🌐 For the face that he beheld was not the face of MacAllister, the fly-cop.
1935		A.J. Pollock Und. Speaks n.p.: Fly bull, a detective.
1940		(con. 1888–1910) H. Asbury Gangs of Chicago (2002) 125: Detective Clifton Wooldridge, known to the underworld as ‘that damned little fly-cop.’.
1949		Monteleone Criminal Sl. (rev. edn) 88: fly bull A plain cloth officer. [...] a detective.
1962		Ragen & Finston World’s Toughest Prison 799: fly cop [...] fly dick – A detective.
1965		R.L. Pike Mute Witness (1997) 91: A cop. A chinzy low-pay fly-cop.

2. an alert or experienced police officer.

1859		Matsell Vocabulum 34: fly-cop. Sharp officer; and officer that is well posted; one who understands his business.
1865		Leaves from Diary of Celebrated Burglar 5/2: It being market day, we concluded to give hull a ‘dressing’ but we had not ‘worked’ it long before the ‘fly-cops’ were out in quest of us.
1935		A.J. Pollock Und. Speaks n.p.: Fly cop, a clever detective.