grass on

grass on (third-person singular simple present grasses on, present participle grassing on, simple past and past participle grassed on)

1. (UK, slang) To betray by informing on.
   • 2008, Danny King, School for Scumbags:

     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.

• Garsons, sarongs