If you watch movies about the London criminal subculture or catch a fair amount of British crime drama on television, you've probably come across the word grass in various uniquely British uses. Though over time, you may pick up the meaning from the context that surrounds it, how the word grass came to be used in these particular ways is a bit of a puzzle.
Grass as a Noun
A grass is criminal who informs on his associates. By extension, it's used by anyone who informs on another over bad or criminal behaviour. For example, a teacher trying to discover who is bullying another student might come up with a wall of silence from other teens who don't want to be seen as being a grass.The expression "supergrass" (also the name of a British band) arose during the Irish "troubles" and was used to describe IRA members who were informers. Today the term Supergrass is still used to describe someone with information about major criminal organizations.
Grass as a Verb
To "grass" on someone or some group is to be an informer. So if a grass is an informer, to grass or grassing describes the act of informing. When you grass on someone or something you are not only filling the role of informer but also of betrayer. That's because grassing carries with it the idea that the "grass" is giving information about his close associates (or her's actually, though grass in this sense is rarely used by women and girls). If you witness a crime that has nothing to do with anyone you know and then give evidence to the police, you are just a witness, not a grass; you are giving evidence, not grassing. Grassing is about betraying your peers by acting as an informer.
The use of grass and "to grass" in this way arose as street argot in the London criminal subculture and dates back to the early part of the 20th century. There are two popular theories about how this came about - one is that it derived from the expression snake in the grass which actually dates all the way back to the Roman writer Virgil. A more likely possibility, since the usage first arose in London, is that it is rhyming slang for to shop or shopper, which have similar meanings (to shop someone is to turn them in to the police).
Follow, if you can, the twisted route through rhyming slang - Policemen are often called "coppers" in British slang. That becomes "grasshoppers" in London slang. Someone who turns his pals, or their information over to the police "shops" them to the authorities becoming a "grass shopper", simplified to "a grass". Maybe that's where the word comes from and maybe its origins will remain shrouded in mystery.