Green’s Dictionary of Slang

1. an informer.

1924		E. Wallace Educated Evans (1932) 73: ‘Snouts?’ said ‘The Miller,’ elaborately puzzled. ‘What is a “snout”?’ ‘It’s a phrase used by low people [...] a man who keeps the police informed on criminal activities.’.
1938		F.D. Sharpe Sharpe of the Flying Squad 189: A ‘sneak’ or ‘snout’ is looked upon more or less as a leper in the Underworld.
1957		P. Beveridge Inside the C.I.D. 11: The detective outside had a tip from one of his informants, the men known as ‘stool-pigeons’ or ‘snouts’.
1972		P. Fordham Inside the Und. 36: He didn’t want any snout putting the Law on a watch.
1989		Beano Comic Library No. 176 51: We need a grass Gnasher – you know, a snout – an informer.
1992		G. Disher Paydirt [ebook] [H]e’d built himelf a good network of snouts.
1995		B. James Detective is Dead (1996) 9: You were acting on a tip from an informant, a so-called grass, or tout or snitch or snout, were you not?
2004		J. McCourt ‘Vilja de Tanquay Exults’ in Queer Street 394: A mistake on my part, luv, to give the snouts / A sneak of the ending.

2. (Aus.) the police.

2010

L. Redhead Thrill City [ebook] You know what I like about you [...] You don’t run whingeing to the snouts.