Green’s Dictionary of Slang

1. (US Und.) a plain-clothes police officer on observation duty.

[1869		Bryant’s Minstrels in N.Y. Herald 18 Nov. 8/5: Shoo, fly, don’t bodder me].
1893		Harper’s Mag. 87 July 304/2: Others, again, were the temporary phrases which spring up [...] and flourish unaccountably for a few months, and then disappear forever, leaving no sign; such as shoo-fly in America and all serene in England.
1909		L.F. Fuld Police Administration 456: The employment of spies, who are usually called ‘shoo-flies.’ These spies may be patrolmen in citizen’s clothes, uniformed roundmen sent out from headquarters, superior officers in citizens clothes or private citizens [DA].
1926		Maines & Grant Wise-crack Dict. 14/1: Shoo-flies – Detectives in parlance of the road.
1936		Mencken Amer. Lang. (4th edn) 566: Shoo-fly afflicted the American people for four or five years, and ‘I don’t think,’ aber nit, over the left, good night and oh yeah were scarcely less long-lived.
1938		A.J. Liebling ‘The Jollity Building’ in Just Enough Liebling (2004) 250: The shooflies are working in our territory [...] and we can’t let a big brawl like this run.
1946		W.L. Gresham Nightmare Alley (1947) 77: Shoo flies, kid. Cops.
1975		E. Torres Carlito’s Way 25: So now you got some shoo-fly checking out the P.
1982		C. Castleman Getting Up: Subway Graffiti In N.Y. 172: [...] beakies or shooflies [MTA inspectors].

2. an informer.

1892

L.W. Moore His Own Story 287: My ‘shoo fly men’ were watchful and diligent, and not a Pinkerton man could drop into Toronto or leave the town without the ‘tip’ being given me.

3. attrib. use of sense 1.

1930		E.H. Lavine Third Degree (1931) 161: The local or borough inspector will send out a ‘shoo-fly’ officer in plain clothes to get him.
1975		E. Torres Carlito’s Way 14: [F]iguring the phones in the poolroom or the bar was tapped and Moran’s name would get on some shoo-fly tape.

4. an undercover police officer who spies on his colleagues; thus shoo-fly duty .

1877		Daily Graphic May 1: A ‘shoofly’ is the term applied by a policeman to another officer who is detailed to watch him.
1897		N.Y. Times 15 Aug. 5: [Roundsmen will go on] what is known in police parlance as ‘shoo-fly’ duty. They will go about the city in plain clothes on the look-out for delinquent policemen.
1904		N.Y. Eve. Post 4 June 2: Lack of discipline is still apparent in the Police Department, notwithstanding the restoration of the ‘shoofly’ system [DA].
1931		(con. 1900s) C.W. Willemse Behind The Green Lights 97: A force of ‘shoo flies’ – roundsmen in civilian clothes – were sent out regularly from Headquarters to sweep into a precinct and look over the men. The ‘shooflies’ were expected to make complaints – or ‘didos’ as the policemen called them.
1955		Q. Reynolds Police Headquarters (1956) 49: ‘Shoo-flies’ were men sent out of Headquarters in plain clothes to check up on the men on the beat.
1958		M. Berger in N.Y. Times 20 Oct. 34/1: A spying department superior is still a shoofly.
1967		J.H. McNamara ‘Uncertainties in Police Work [etc.]’ in Bordua Police 182: Patrolman believed the shoo-flies were required in a given time period to file a minimum number of complaints against patrolmen.
1973		P. Maas Serpico 173: Walsh’s investigators—or ‘shooflies’—caught countless cops in minor violations of the department’s rules.
1987		M. McAlary Buddy Boys 181: [I]f the shoofly—some supervisor trying to check up on us—came into the park looking for us, we could see him coming.
1994		M. McAlary Good Cop Bad Cop 87: The shooflies, as internal investigators were nicknamed by rank-and-file patrolman [sic], were easily spotted by Dowd and Eurell.

5. attrib. use of sense 3.

1975		E. Torres Carlito’s Way 14: Moran’s name would get on some shoo-fly tape.
1986		C. Stroud Close Pursuit (1988) 238: He laid a brutality charge on Wolfie, along with excessive force [...] so he’s gonna have the shoo-fly brigade sitting on his head.

6. a drug used to deal with troublesome drinking club customers.

1952

(con. 1920s) G. Fowler Schnozzola 63: They called the Mickey a ‘Shoo Fly’. [...] Being a laxative for a horse, you can imagine what it does to the anatomy of a human being if he gets boisterous or rough.