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"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?

  • Isn't it to "grass on someone"? EDIT Just checked the dictionary. Indeed "up" is used in the transitive verb. – Mari-Lou A Jun 14 '16 at 13:15
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    I don't have any cites for this (beyond the popular song en.wikipedia.org/wiki/Whispering_Grass ) , but I could easily conceive of a poetic or folkloric idea that the blades of grass are tongues which speak, if only we listen. Like the song says 'why do you whisper, green grass? Why tell the trees what ain't so...' – Spagirl Jun 14 '16 at 13:18
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    @Spagirl If someone can find a citation for your theory then that is an explanation I'd much rather believe than simple rhyming slang. – Ste Jun 14 '16 at 13:21
  • To grass is also to have work on the side (Printing slang), when you are a grass you're selling out someone, you work for the other side ,so to speak. – P. O. Jun 14 '16 at 13:24
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    @Mari-LouA "grass on" and "grass up" are both used. – A E Jun 14 '16 at 13:54
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The Phrase Finder explores three different possible origins in the following extract:

Grass up:

  • In 2005, British newspapers picked up on a story about a burglar who had stolen cash, jewellery and an African Grey parrot from a house near Hungerford, Berkshire. David Carlile, widely described in the press as 'feather-brained', explained to the police that he knew that African Greys could talk and he didn't want the bird to 'grass him up'. Presumably, had the parrot been a Norwegian Blue, he would have left it to pine for the fiords.

First assumption:

  • 'Grassing up' has been a commonly used expression in the UK since the mid 20th century, but is less common elsewhere. The first known use of 'grass' in that context is Arthur Gardner's Tinker's Kitchen, 1932, which defined a grass as "an informer".

  • Grass was a well-enough established word in the 1980s to have spawned 'supergrass', that is, a republican sympathiser who later 'turned Queen's evidence' and informed on the IRA, and which gave the Brit-pop band Supergrass their name in the 1990s.

Second assumption:

  • There is another route to the word and this is via rhyming slang. Farmer and Henley's 1893 Dictionary of Slang defines 'grasshopper' as 'copper', that is, policeman. The theory is that a 'grass' is someone who works for the police and so has become a surrogate 'copper'. The rhyming slang was certainly believed in 1950 by the lexicographer Paul Tempest, when he wrote Lag's lexicon: a comprehensive dictionary and encyclopaedia of the English prison to-day:

    • "Grasser. One who gives information. A 'squealer’ or ‘squeaker'. The origin derives from rhyming slang: grasshopper - copper; a 'grass' or 'grasser' tells the 'copper' or policeman."
  • That comes only a few years after the term grass was coined and there seems little reason to doubt it as the derivation. The original users of the term 'grass up' were from the London underworld and would have certainly been better acquainted with rhyming slang than the works of Virgil.

Third assumption:

  • Some have also theorised that the term 'shop', meaning 'give information that leads to an arrest', derives from the same source, that is, that, as 'grass' derives from 'grasshopper', then so does 'shopper'. The earliest known use of shop in that context dates from around the same time as the emergence of grasshopper. The issue of the magazine Tit-Bits for May, 1899 includes:

    • "[He] volunteered for a fiver to 'shop' his pals."
  • As far as we know, African Greys don't go shopping.

The second and third hypotheses are supported also by the Word Detective:

  • The use of “grass” as British slang for a police informer dates back to the 1930s, and is apparently a short form of the slang term “grasshopper,” meaning the same thing. “Grasshopper” itself is rhyming slang (“a secret language” in which words rhyme with a hidden meaning) for either “copper” (i.e., a police officer) or “shopper,” one who “shops” (sells) information to the police.
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  • Doesn't shop just derive from copshop? – Araucaria - Not here any more. Jun 14 '16 at 13:28
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    No lesser authority than the 1811 Dictionary of the Vulgar Tongue, a dictionary of ‘Buckish Slang, University Wit, and Pickpocket Eloquence’ by Captain Grose, gives the following. SHOP. A prison. Shopped; confined, imprisoned. From which I would take that the original sense of ‘shopping’ is not just to grass someone up, but to do so to the effect that they are jailed. – Spagirl Jun 14 '16 at 13:42
  • I like the fact that accounting for "shop" as part of "cop-shop", distances "grass" from the rhyming slang theory. – Ste Jun 14 '16 at 13:57
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    Another possible origin that I'd heard (mentioned, and dismissed, here), is that "Grass in the park" is rhyming slang for "Copper's nark". – psmears Jun 14 '16 at 20:00
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I always thought it was just short for "snake in the grass".

Edit:

I can't comment yet, and I'm not sure if it's acceptable to respond to comments below, so if not I guess it can be edited or removed.

I disagree that the top sentence of this post was not an answer, when the question asks if there are "any other possibilities aside from the rhyming slang version above?", and I gave exactly that: another possibility.

Having looked into this a bit more I see even less reason to believe "grass" derives from "grasshopper".

The Phrase Finder page (which also mentions "snake in the grass" as a possibility), cited in the other answer, states the first known use of the term "grass" (informant) was 1932, whereas the term "grasshopper" (policeman) dates from the late 19th century.

Snake in the grass being first used by Virgil is not really relevant here; what's more relevant is that it was used in American underworld circles for informant around the same time grass started to be used for informant in British underworld.

The magazine Detective Story even ran a serialised story entitled The Snake In The Grass throughout 1925, so it's probable that the term appeared in other American crime fiction of the time; perhaps even some films.

It seems more likely to me that British villains derived a word for an informant from a popularised underworld term for an informant used at the time, than from a slang word for policeman coined 40 years earlier.

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    This is an interesting possibility. Have you found any sources which countenance it? – GoldenGremlin Jun 14 '16 at 18:29
  • A snake in the grass is a much older saying, I could not find a clear connection with the more recent meaning of "grass up", but the two meanings are somehow related. - We owe the proverbial saying “snake in the grass” to the Roman poet Vergil (70-19 B.c.). " In the third Eclogue is the line Latet anguis “A snake lurks in the grass.” - *The earliest English translation was Edward Hall’s Chronicles (1548): The Union of the Two Noble and Illustre Famelies of Lancestre and Yorke: “But the serpent lurked vnder the grasse, and vnder sugered speache was hide pestiferous poyson.” – user66974 Jun 14 '16 at 19:39

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