A grass

A grass, a snout, copper's nark, a stool pigeon, slang terms for a police informant. Snout and nark are slang terms for nose, the latter being of Romany Gypsy origin. Stool pigeons were essentially pigeons that were tied to a stool type frame to attract game birds that could then be shot, this started in 19th century America and the term "stool pigeon" was used as a term for someone who lured someone else into a trap, during the wars it was used as a term for spies and now more commonly as a police informant.

Grass has its origins in Cockney rhyming slang and here's a clue

It's a grasshopper, now the rhyme part is either for copper (policeman) or shopper (as in "one who shops"). To 'shop' was also used as a term for informing ("I've shopped him to the police"), "shop" was a 16th century slang term for prison, originally the verb to shop was used as slang for putting someone in prison and evolved into having someone put in prison.

So now you know, remember where you heard it first, just don't rat me out to the old bill.