It means to help the authorities by “ratting out someone”

It’s always a criminal snitching on a criminal in the cases I reference. Often considered going against their “moral code.”

to the peelers …


✌🏻

Don't fuck me over.

and in my reading and understanding, it remains fairly specific to that.

Don't grass me up. And it's a cop talking to an informant or in interview rooms.

And I took it for telling them not to lie or mess around wasting their time.

and in my hearing 'grass' almost invariably means to inform or 'snitch' - or alternatively the informer or snitch.
but if that's the way you heard it - so be it.
language is a moving target. yes?

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And then there's a "supergrass," lower than a snake in the grass,
an informant during The Troubles.