grass on

• [俚语](向警方)告发(同伙罪犯)：

  The police would never have caught the jewel robbers if the prisoner had not grassed on them and told them where they were hiding.

  要不是这罪犯向警方告发了抢劫珠宝的同伙，并说出他们的藏身之地，警方不可能抓到这伙劫匪。