rat somebody out (to somebody) (especially North American English) (also rat on somebody British and North American English) (informal, disapproving) to tell somebody in authority about something wrong that somebody else has done

rat out v. To expose or reveal incriminating or embarrassing information about someone: The newspapers ratted out the people who were involved with the scam.

It means that you told the truth about something they did wrong. For example, a thief might rat out his partner to the police in exchange for a smaller punishment.

From Longman Dictionary of Contemporary English rat somebody ↔ out American English informal if someone rats you out, they are disloyal to you, especially by telling someone in authority about something wrong that you have done You can’t rat out your teammates. → rat → See Verb table Examples from the Corpus rat out • Magic shop in Plantation, the rat stock sells out nearly before it hits the shelves.