grass on

listen to the pronunciation of grass on
Englisch - Türkisch
(Konuşma Dili) ispiyonlamak
Polise suç ihbarında bulunmak
Englisch - Englisch
To betray by informing on

What I was pissed off about, though, was him telling everyone that he'd only grassed on me because I'd grassed on him first, which was a total lie.

(British informal) inform the police of criminal activity or plans
grass on