Grass
British slang from c. 1920 for a police informer. It can also be used a verb as in to grass on someone. It is believed to be an abbreviation of grasshopper, rhyming slang, which dates from the late 19th century. Grasshopper had two rhyming slang connotations, which are both contenders for the origin. The first is grasshopper/copper, meaning policeman, with the inference that the informer is acting as a surrogate or undercover police officer. The second is grasshopper/shopper, as in someone who ‘shops’ or informs on someone else. Grasser, one who grasses is known from c.1940. See also Snitch and Nark.