Q From Bill Brown: Today I came across the phrase grass them up which I gathered from the context means to turn in to the authorities. Searching the web confirms this, but I didn’t come across any explanation of its origins. I have faith you can explain this phrase. [Bill Brown
A It’s good that you have such faith in my etymological detective work, Mr Brown, but I doubt whether in this case I’ve tracked this well-established slang term to its origin.
To grass in British slang is indeed to inform on a person to the authorities; a grass is an informer. The noun starts to appear in print in the 1920s and the verb a few years later. We’ve since had grasser in the same sense; in the 1970s supergrass appeared for a police informer who implicated a large number of people at one go.
It has been proposed that grass is from snake in the grass, a treacherous person or a secret enemy. This echoes the ancient idea that snakes are perfidious creatures, a view that famously appears in the Book of Genesis. I’ve also come across a curious argument that it derives from grass in the park, rhymingly a copper’s nark. (Nark is known from the last third of the nineteenth century and comes from Romany nak, a nose, that is, somebody who sticks his nose into others’ affairs or sniffs out information; it’s no relation to the US narc, short for narcotics officer). We’re quite sure that neither of these ideas is correct.
Instead, the experts point to grass as being a short form of grasshopper. We may pass over the latter’s earliest slang sense of a waiter in a tea-garden — which brings to mind an overworked server bounding from customer to customer — and concentrate instead on the meaning first recorded by John Farmer and W E Henley in volume three of Slang and its Analogues in 1893: a policeman, by rhyming slang a copper.
Earlier writers on slang assumed that grasshopper was extended to refer to informers because of their police connections. More recent writers are less sure.
The experts are instead favourably disposed towards another slang term, to shop. This dates from the sixteenth century, when it meant to imprison (it comes from the noun shop, which in low slang then referred to a prison). By the early nineteenth century it had taken on the sense of providing the evidence by which a person was sent to prison, hence inform. A grasshopper might therefore have more obviously been a shopper, not a copper. Shopper begins to be recorded in the sense of an informer around the time grass starts to appear.
So far as I’ve been able to find out, there’s no direct evidence for either copper or shopper. The current predisposition among slang lexicographers to prefer the latter is basically that it has a more direct semantic association with grass via grasshopper.