From Longman Dictionary of Contemporary Englishrat somebody ↔ out American English informalif 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.• Giant amphipods, the size of rats, appear out of nowhere to nibble on the remains.• When they blast, the rats jump out of the sewers and then dive back in again.• Sometimes the bravest rats ran out on to the floor or across the tops of the pictures. 