In British slang, a "grass" is an informer, specifically someone who informs the police or authorities about others, often betraying a group or individuals they know
. The term "to grass" means to inform on someone or "to rat someone out"
. This usage dates back to at least the 1920s and is believed to have originated as a shortening of the rhyming slang "grasshopper," which itself is linked to "shopper," meaning an informer
. Another suggested origin is from the phrase "snake in the grass," referring to a traitor
. Thus, calling someone a "grass" in British slang is accusing them of being a snitch or police informant.
