grass on
phr. v..phr. v.. to tell the police or someone in charge about something bad that another person did.
phr. v.. to inform an authority figure or the police about a person's illicit or prohibited activities; functions as a transitive verb of betrayal.
He refused to grass on his friends.
The gang members were worried that one of their own might grass on them to the police.
Despite the pressure from investigators, the witness remained silent, unwilling to grass on the individuals who had orchestrated the scheme.
usually followed by the person being reported; carries a strong negative connotation of being a 'snitch'.
this is a highly regional British term; compare it with the American 'snitch on' or 'rat out' to help students understand the cultural equivalent.
He grassed his brother.He grassed on his brother.the particle 'on' is required to indicate the person being reported.