grass:
(plural: ‘grasses‘; pronunciation: the letter ‘g’ in this word rhymes with the letter ‘g’ in the word “garden” — for the right pronunciation of this word, please click here)
(mostly in British English) a person (may be a criminal him/herself) who informs on the activities or whereabouts of other criminals;
a police informer,

a grass passing on information to a police officer
The entire family had a shock when they came to know that young Peter, the one very much loved by everyone, was a grass, working for the district police!
A terrorist group grass is lucky when he’s caught by the police because the police protect him from the terrorists to get information, but a police grass is a goner if he’s caught by the terrorists because terrorists mercilessly kill those who cheat them.
There are more grasses in the police department who inform the underworld Mafia families about the police plans than there are for the police who inform them of the Mafia activities.
When the information the grasses pass on to the police is worthy, the grasses are given money and other privileges, such as writing off previous charges, and honour them with the title ‘super-grasses’!

(common grass plants)
Note 2: ‘Grass’, always with a capital ‘G’, is often found in the names of people, e.g. Gunter Grass, Grace Grass, Vincent Grass, etc. For an article about the origin of this word as a surname, please click here.
