Green’s Dictionary of Slang

1. a detective.

1886		W. Newton Secrets of Tramp Life Revealed 9: Jack or Teck ... Detective.
1925		Smith’s Wkly (Sydney) 11 Apr. 13/5: I’m in a capital city and none of the jacks know me.
1931		Eve. News (Rockhampton, Qld) 27 May 3/1: Other curious names in everyday use' among criminals [are] ‘jacks’ (detectives), and ‘dogs’ (police shadowers, who dog the heels of suspects).
1935		Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 6: Jack: Plain clothes officer.
1936		Queenslander (Brisbane) 2 July 4/4: Into the courtroom through a heavy gate / I went. And on the Bench old Bleareyes sate. / A ‘Jack’ steps up and says his piece, / And then His Nibs beefs out my fate.
1941		F. Sargeson ‘That Summer’ in Stories (1982) 194: We all had to stand there with a crowd of jacks in plain clothes standing round, and one in uniform called out our names and said what we'd been picked up for.
1943		Baker ‘Influence of American Sl. on Australia’ in AS XVIII:4 256: There’s two Jacks on me hammer.
1950		P. Hoskins No Hiding Place! 191/1: Jacks. Detectives.
1965		(con. 1940s) B. Behan Confessions 20: Jesus, that’s a funny thing. I’m a Jack myself.
1968		J. Alard He who Shoots Last 139: A carload of jacks picked me up [...] Dey wanted some information.
1976		D. Ireland Glass Canoe (1982) 15: ‘Your old man isn’t going to like this,’ one of the Jacks said.
1980		J. McClure Spike Island (1981) 83: Black and white, a lot of Uniform work is, not bloody grey like the jacks have to deal with.
1982		G. Newbold Big Huey 249: Jack (n) 1. Detective.
1991		M.B. ‘Chopper’ Read Chopper From The Inside 197: The Pom was busy cutting toes, / The jacks sat back picking their nose.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 94/1: jack n. 2 a detective, a (plain-clothes) police officer.
2002		S. Maloney Something Fishy (2006) 231: A homicide cop called Kevin Hayes. One of the many jacks I’d met that morning.
2012		A. Nette ‘Chasing Atlantis’ in Crime Factory: Hard Labour [ebook] ‘He’s a fucking jack,’ said Slade.

2. a police officer.

1889		Clarkson & Richardson Police! 255: ‘Smoke Jack’ is the name given to the policeman whose special duty it is to note breaches of the Smoke Nuisance Abatement Acts.
1909		J. Ware Passing Eng. of the Victorian Era 158/2: Jack (Lambeth, 1865–72). A policeman – quite local.
1918		Aussie (France) Sept. 11/2: We’d choose some big bonza towns where there are no Jacks to hunt us down and where there’s no place Out of Bounds.
1936		K. Mackenzie Living Rough 275: You should ’ave seen the fun when the Jacks came [...] I jumped down a fence, I was away to ’ell.
1938		Press (Canterbury) 2 Apr. 18: ‘The jacks’ are policemen.
1941		F. Sargeson ‘That Summer’ in Coll. Stories (1965) 192: We had to stand there with a crowd of jacks in plain clothes standing round.
1968		J. Alard He who Shoots Last 1: The jacks will make it hot for a while.
1984		B. Ellem Doing Time 191: Jack: a policeman.
1984		Smiley Culture ‘Cockney Translation’ 🎵 What cockney call a Jack’s we call a Blue Bwoy.
1989		J. Morton Lowspeak 78: Jack [...] 3. policeman, or detective. When the Liverpool police were investigating the Wallace murder in 1920 [...] they were called Springheeled Jacks [...] Springheeled Jack was a well known Victorian melodrama.
1992		G. Disher Paydirt [ebook] The nearest cop shop is an hour away. I never saw a single jack .
2000		N. Griffiths Grits 326: If anything was ever knocked off in the area the fuckin jacks’d be straight round me bleedin ouse.
2003		N. Griffiths Stump 36: Fuckin place is crawlin with jacks.
2007		L. Redhead Cherry Pie [ebook] ‘I’ve hated jacks since I got bashed after a demo’.
2023		A. Nette Orphan Road 184: ‘What can you tell me about Hardigan?’ ‘An ex-Jack’.

3. constr. with the, the Military Police.

1919		Aussie (France) XII Mar. 14/1: [of a military policeman] J’s for the Jacks — keep out of their way / If you don’t like C.B. and a thirsty pay-day.
1924		(con. WWI) A.G. Pretty Gloss. of Sl. [...] in the A.I.F. 1921–1924 (rev. t/s) n.p.: jacks. Military Police.
1925		(con. WWI) Fraser & Gibbons Soldier and Sailor Words 130: Jacks, The: Military Police.
1930		(con. 1914–18) Brophy & Partridge Songs and Sl. of the British Soldier 131: Jacks. — Military Police.
1943		Sydney Morn. Herald 11 Dec. 7/3: The Australian military police [...] invariably known as ‘jacks’.