to warn someone secretly about something that will happen, so that they can take action or prevent it from happening

[ + that ] Somebody must have tipped the burglars off that the house would be empty. 一定是有人给盗贼通风报信，告诉他们房子里到时候会没有人。

The robber was caught when someone tipped off the police. 有人向警方报信后，抢劫者即被抓获。