"Grass", in British English, can be used as a verb or a noun to describe a police informer or the actions of said informer. Oxford gives:
noun: British informal, A police informer.
verb: British informal, Inform the police of someone’s criminal activities or plans
The term "grass" has been widened in its usage to also mean "reporting someone to an authority figure":
Don't eat those biscuits or I'll grass you up to mother!
Oxford suggests that the term may have come from rhyming slang, namely grasshopper/copper. Etymonline gives nothing.
Can one of our resident etymologists provide an insight into when and where this first came into use and whether there are any other possibilities aside from the rhyming slang version above?