To grass on somebody, or grass up somebody (or grass somebody up) is to tell someone in authority, especially the police, about something bad that somebody has done.
Informal British / UK English.
grass (verb) to tell the police about something that somebody has done wrong.
grass (noun) a police informer. British/UK English.
Examples of use:
1. She grassed on us to the local police.
2. Which one of you grassed me up to the police?!
3. Please don’t grass him up – he won’t do it again.
|present simple||grass on/up and grasses on/up|
|-ing form||grassing on/up|
|past simple||grassed on/up|
|past participle||grassed on/up|
Practise your English and write your own sentence using ‘grass on’ or ‘grass up’
Image © William Warby