Green’s Dictionary of Slang

1. a detective or police officer; often in pl.

1886		Glen Innes Examiner (NSW) 26 Oct. 6/1: ‘I remember ’im playin’ the Demon in “The Crimes of Paris,” and now he’s doin’ a feller who gets “jugged” for a crime he never did.’.
1889-90		D. Sladen in Barrère & Leland Sl., Jargon and Cant I 304/1: Demons (Australian), prison slang for police. ‘The demons put pincher on me’, I was apprehended.
1890		‘Welsher’s Confession’ in Seal (1999) 143: Just then a ‘demon’ came up, and I squared him with two quid.
1902		Bulletin (Sydney) 27 Sept. 29/2: Fur there’s whips er shine women, / An’ liquor an’ spons, / When yer lurk / Is ter work / Fur the demons an’ johns.
1906		Sun. Times (Perth) 16 Dec. 1/1: The army of guns and tugs [...] daily rub shoulders with the drowsy demons.
1910		Bulletin (Sydney) 27 Oct. 14/1: Nosey was speaking to a detective about the case alongside the Darlinghurst Gaol before an arrest had been made. The demon says: ‘This is a terrible thing, Bob.’.
1922		Smith’s Wkly (Sydney) 2 Dec. 18/2: He was just biting him for a tenner till the bank opened, when a big, fat ‘demon’ [...] grabbed him.
c.1936		Anonymous ‘The Dying Bagman’ in Seal (1999) 96: The coppers that wait at Rockhampton, / The demons that wait at Yatrell.
1941		K. Tennant Battlers 96: When five men had been arrested that same afternoon, Adelaide had been triumphantly certain that the shearers were ‘demons’, or plain-clothes detectives.
1941		F. Sargeson ‘That Summer’ in Stories 195: [W]hen [...] I was beginning to get the dingbats through being there [i.. in a cell] so long on my own I was taken out by the two demons who’d picked me up.
1949		Cessnock Eagle (NSW) 4 Oct. 4/2: They took a picture to identify me. / Then forty demons said ‘just nod your head and you’ll be free’ / So I pleaded ‘Guilty’ just like a ‘gay’ / Now I’m a boarder out at Long Bay.
1950		I. Hamilton Till Human Voices Wake Us ms 16: [A] big demon walked into Joe’s cell with his coat off. How’re you feeling, he says? Crook, Joe says, and pretends to start puking in a corner. The demon walks up and down the cell. Pity, he keeps saying, I just feel like a round or two.
1968		S. Gore Holy Smoke 88: What about if it’s a demon in plain clothes?
1973		(con. 1930s) F. Huelin ‘Keep Moving’ 53: Basher Bill O’Malley, in charge of a dozen railway demons, was determined to enforce the edict.
1982		(ref. to 1880s+) G. Newbold Big Huey 247: demon (n) Detective. In the sense of an ordinary policeman, Australian since about 1880. As a detective, a New Zealand specialist usage since about 1932.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Demons. Police, usually plain clothed. Extension of ‘Ds’.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 54/2: demon n. 1 a police officer, esp. a detective.
2003		McGill Reed Dict. of N.Z. Sl.
2010		W. Ings ‘Trolling the Beat to Working the Soob’ in Int’l Jrnl Lexicog. 23:1 69: Terms for the police found both in boobslang and on the street include Demon, headlice (po-LICE), pig and filth.
2018		G. Gilmore Base Nature [ebook] ‘It’s freezing in here.’ ‘Must be all the demons’.

2. (N.Z. prison) an unmarked police car.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 54/2: 3 an unmarked police car. [such a car is generally driven by a detective].