Green’s Dictionary of Slang

johnny n.2

[john n.1 ]

a police officer; a prison guard.

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[UK](con. 1840s–50s) H. Mayhew London Labour and London Poor II 154/1: The ‘Johnnys’ on the water are always on the look out, and if they sees any on us about, we has to cut our lucky.
[UK]Besant & Rice By Celia’s Arbour I 80: We might run up and down the slopes or on the ramparts [...] without rebuke from the ‘Johnnies,’ the official guardians of the walls.
[UK]Graphic 30 Jan. 130/1: Constables used to be known as johnny darbies, said to be a corruption of the French gensdarmes, and they are still occasionally called johnnies [F&H].
[Aus]W.T. Goodge ‘Great Aus. Slanguage’ in Baker Aus. Lang. (1945) 117: A policeman is a johnny / Or a copman or a trap.
[US]J. Flynt World of Graft 23: I’ll bet you ten to one that you an’ me could take a walk now through the streets an’ see a lot o’ graftin’, an’ yet those Front Office Johnnies ’ll make the same trip an’ go home empty-handed.
[US]C. Himes ‘Prison Mass’ in Coll. Stories (1990) 166: He’d been walking down Market Street when he first passed the cop. He had looked at the johnny and the johnny had looked at him.
[US]Goldin et al. DAUL 111/1: Johnny. (South) A prison guard; a chain-gang guard.
[UK]S. Berkoff West in Decadence and Other Plays (1985) 117: Watch out there’s a johnny.

In compounds

johnny-be-good (n.) [play on rock song ‘Johnny B. Goode’ (1958)]

(US black) the police.

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[US]E. Folb Runnin’ Down Some Lines 66: The characterization of the police [...] connotes such perceived attributes as [...] hypocritical morality (divine right, deputy do-right, do-right, Johnny-be-good).
johnny law (n.) [law n. (3)]

(US) a police officer, usu. male.

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[US]N. Klein ‘Hobo Lingo’ in AS I:12 651: Johnny Law — detective.
[US]J.T. Farrell Gas-House McGinty 121: He would train the automatic on the guy while Mame called Johnny Law.
[US] ‘I’ll Gyp You Every Time’ in C. Hamilton Men of the Und. 178: The ‘patch’ [...] had taken care of everything as far as Johnny Law went.
[US]‘Hy Lit’ Hy Lit’s Unbelievable Dict. of Hip Words 24: johnny law – The police.
[UK]S. Berkoff East in Decadence and Other Plays (1985) 64: The mad Irish had gone to the johnny law with some mad tale of rape.
[UK]S. Berkoff West in Decadence and Other Plays (1985) 106: [It] doth excite old johnny law to exercise his tool on us.
[US]Eble Campus Sl. Apr. 5: johnny/johnny law – police officer.
[US]‘Touré’ Portable Promised Land (ms.) 42: When Johnny Law showed up it was too late.
[US]F. Bill ‘Old testament Wisdom’ in Crimes in Southern Indiana [ebook] ‘We was damn good friends, Jacque.’ ‘Until you decided to become Johnny Law’.
[US]S.A. Crosby Blacktop Wasteland 107: Some over eager Johnny Law decided to racially profile them.