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