What it means
To grass someone up means to snitch on them, dob them in, or go running to a teacher, the police, or whoever's in charge. It's proper accusatory slang and usually comes with a strong whiff of betrayal, especially when it was meant to stay between mates. You can grass someone up, or call someone a grass.
Usage examples
"Who grassed me up to the headteacher about the fire alarm prank? Don’t look at me, I was bunking off behind the bike sheds with a sausage roll, swear down"
"Somebody grassed me up to the headteacher about the chips behind the bike shed, three guesses who it was and the third guess wears glasses and sits next to the heater."
"Don't grass me up to the missus about the kebab on the way home, mate, that stays between us and the bloke at the chippy who saw me eat it on the kerb."
"Oi, don't grass me up for jumping the barrier, the inspector was already giving me the evil eye."
"He grassed us up for sneaking out at lunch, then acted like he was doing everyone a favour."
Where it comes from
In British slang, grass for informer is commonly linked to rhyming slang. Grasshopper rhymed with copper, meaning police, and the phrase got clipped down over time until grass stuck. From there, grass up took off as the everyday verb for dobbing someone in to teachers, police, or any other authority figure.
Other ways to say it
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