Green’s Dictionary of Slang

1. a uniformed police officer.

1867		Nat. Police Gaz. (NY) 12 Oct. n.p.: A ‘flatty-cop’ [...] discovered him ’working’ [...] and ‘took him in’.
1881		Newcastle Courant 25 Nov. 6/5: Cease yer barrikin’, don’t yer see flatty’s a’coming.
1892		W. Norr Stories of Chinatown 40: That sucker had actually followed us [...] with the flatty and nailed Fritz. [Ibid.] 50: The flatties closed in on me to put on the nippers.
1900		Flynt & Walton Powers That Prey 51: The ‘flatties’ in uniforms surrounded the place.
1909		‘The Lang. of Crooks’ in Wash. Post 20 June 4/1: [paraphrasing J. Sullivan] The policeman is [...] a bull, a beeby, a fatty [sic] finger or a harness bull.
1915		O.O. McIntyre Day By Day in New York 6 Apr. [synd. col.] He began work as a ‘flattie’ or patrolman down around Fulton Street.
1922		Joyce Ulysses 469: The squeak is out. A split is gone for the flatties.
1927		J.M. Walsh Man behind Curtain (1931) 105: I’ll admit I sang a lot [...] and I’ve got a nidere I kissed a ‘flattie’.
1930		‘Leslie Charteris’ Enter the Saint 67: All clear. The flattie passed ten minutes ago, and his beat takes him half an hour.
1930		Eve. Herald (Dublin) 9 Dec. 4/6: Other [underworld] terms include : — ‘Flatty’ (policeman), ‘peach’ (to give away), ‘Peter’ (safe), ‘monkey’ (padlock), ‘stick’ (jemmy), ‘van dragger’ (motor thief), ‘snow’ (cocaine), ‘madam’ (misleading conversation) ‘stir’ (prison).
1931		C.B. Yorke ‘Snowbound’ in Gangster Stories Oct. n.p.: ‘I’m taking orders from no dick [...] not even a private flattie! Beat it!’.
1942		R.L. Bellem ‘Million Buck Snatch!’ Dan Turner - Hollywood Detective Jan. 🌐 Then I massaged that Easter-egg on my skull where that dumb flatty had conked me.
1944		C. Fluck ‘Bubbles’ of the Old Kent Road 41: The laughter and ridicule would bring the flatties (police) and he would be chucked out or knocked off. If the latter, at the court next morning he would plead his own cause.
1953		K. Tennant Joyful Condemned 11: I don’t want any flatties coming round asking questions where I work.
1959		I. & P. Opie Lore and Lang. of Schoolchildren (1977) 395: There are, in the London area, at least thirty nicknames current among boys, and any lad of wit seems to be able to recite a string of them. They include: [...] Flatfoot or Flatty Kipperfoot.
1974		‘P.B. Yuill’ Hazell Plays Solomon (1976) 10: Uniformed flatties were already erupting from the cars.
1977		D. Powis Signs of Crime 184: Flatties Uniform police officers (flatfeet); has a derogatory sense and is thus not used to the face of such an officer.

2. (US und.) a detective.

1909

‘The Lang. of Crooks’ in Wash. Post 20 June 4/1: [paraphrasing J. Sullivan] A detective is an elbow, or a dick, a flatty or a mug.

3. (US tramp) a railroad police officer.

1925

G.H. Mullin Adventures of a Scholar Tramp 26: He is known on the Road not only as ‘dick’ but [...] as ‘bull,’ ‘soft-shoe,’ ‘gum-shoe,’ an ‘elbow,’ a ‘flatty,’ or a ‘mug’.