Green’s Dictionary of Slang

grass n.3

[rhy. sl.; grasshopper = shopper n.; cite 1927 prob. a misinterpretation; Hoskins (1950) suggests ‘snake in the grass’]

1. an informer .

[[UK]Hartlepool Northern Dly Mail 28 Jan. 5/5: I have also learned [...] that a policeman is known as a ‘grass’ — short for grasshopper which rhymes with ‘copper’].
[UK]C.G. Gordon Crooks of the Und. 69: A ‘grass’ is the term for ‘copper’s nark’ in the underworld to-day.
[UK]J. Curtis They Drive by Night 123: I come back to help you, spite of watchew done, and all you do is lie to me and call me a grass.
[Ire]J. Phelan Letters from the Big House 20: Grass is jail-jargon for informer [...] Copper is colloquial for policeman; grass-hopper is rhyming-slang for copper; grass is the abbreviation.
[Ire]J. Phelan Tramp at Anchor 75: A grass is a man who makes a practice of giving information to the authorities about other prisoners.
[UK]F. Norman Guntz 129: You can be a [...] shnide, grass, ponce, bore.
[UK]G.F. Newman Sir, You Bastard 75: Sometimes grasses were a detective’s greatest asset.
[UK]Smiley Culture ‘Cockney Translation’ 🎵 Cockney say grass. We say informer man.
[Aus]B. Ellem Doing Time 190: grass: to inform; an informer.
[Ire]P. Howard The Joy (2015) [ebook] Stupid cunt didn’t even report that. Never does. Doesn’t want to be a grass.
[UK]G. Burn Happy Like Murderers 103: The inspector [...] used Fred West as a ‘grass’ on the local villains.
[UK]C.J. Stone Housing Benefit Hill 94: Kids scream at him in the town: ‘He’s a wrong’un, he’s a grass’.
[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 81/1: grass n. 1 an informer.
[Aus]B. Matthews Intractable [ebook] [T]he Special Purpose Witness Protection Prison, known as the Grass Castle.
[UK]Graffito on bus stop shelter, Tufnell Park, London 27 July n.p.: SEAN (FISH) IS A GRASS.
[Scot]L. McIlvanney All the Colours 11: Whispering grasses. People with the inside dope, the horse’s mouth, on various ministers.
[UK]J.J. Connolly Viva La Madness 44: Turns out the geezer she took up with was a grass.
[Ire]L. McInerney Blood Miracles 129: Pender’s a thief, not a grass.
[Aus]G. Gilmore Class Act [ebook] ‘[I]f journalists started letting on about their grasses, no one would ever grass again’.
[UK]J. Meades Empty Wigs (t/s) 323: ‘I am not a grass... However - your friend Marty is. [...] Marty... he fingered Andy’.

2. (UK und.) information; the act of informing.

[UK]Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 5: Grass: Information.
[Aus](con. 1964-65) B. Thorpe Sex and Thugs and Rock ’n’ Roll 79: ‘Now, one fuckin’ inch of grass ’bout this an’ I’ll do ya’.

In phrases

go (the) grass (on) (v.)

to turn informer (on).

[Ire]Eve. Herald (Dublin) 9 Dec. 4/6: The magistrate [in a London Police Court] was baffled by the words ‘somebody has gone the grass on me,’ which reflects the prisoner’s view that one his associates had informedthe police of his activities.
[UK]Hartlepool Northern Dly Mail 15 Jan. 7/3: ‘Somebody has gone grass on me’ means thata ‘tea-leaf’s (thief’s) associate has given information to the ‘splits’ (detectives).