Green’s Dictionary of Slang

1. an informer .

[1927		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’].
1929		C.G. Gordon Crooks of the Und. 69: A ‘grass’ is the term for ‘copper’s nark’ in the underworld to-day.
1938		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.
1943		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.
1954		J. Phelan Tramp at Anchor 75: A grass is a man who makes a practice of giving information to the authorities about other prisoners.
1962		F. Norman Guntz 129: You can be a [...] shnide, grass, ponce, bore.
1970		G.F. Newman Sir, You Bastard 75: Sometimes grasses were a detective’s greatest asset.
1984		Smiley Culture ‘Cockney Translation’ 🎵 Cockney say grass. We say informer man.
1984		B. Ellem Doing Time 190: grass: to inform; an informer.
1996		P. Howard The Joy (2015) [ebook] Stupid cunt didn’t even report that. Never does. Doesn’t want to be a grass.
1998		G. Burn Happy Like Murderers 103: The inspector [...] used Fred West as a ‘grass’ on the local villains.
2001		C.J. Stone Housing Benefit Hill 94: Kids scream at him in the town: ‘He’s a wrong’un, he’s a grass’.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 81/1: grass n. 1 an informer.
2006		B. Matthews Intractable [ebook] [T]he Special Purpose Witness Protection Prison, known as the Grass Castle.
2008		Graffito on bus stop shelter, Tufnell Park, London 27 July n.p.: SEAN (FISH) IS A GRASS.
2009		L. McIlvanney All the Colours 11: Whispering grasses. People with the inside dope, the horse’s mouth, on various ministers.
2011		J.J. Connolly Viva La Madness 44: Turns out the geezer she took up with was a grass.
2017		L. McInerney Blood Miracles 129: Pender’s a thief, not a grass.
2017		A. Parks Bloody January 6: ‘Fuck off. I’m no a grass.’ McCoy laughed. ‘You’re no a grass? Fuck am I doing sitting here, then?’.
2018		G. Gilmore Class Act [ebook] ‘[I]f journalists started letting on about their grasses, no one would ever grass again’.
2024		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.

1935		Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 5: Grass: Information.
1996		(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’.