Definition of rat on phrasal verb from the Oxford Advanced Learner's Dictionary

 

rat on

 phrasal verb
phrasal verb
 
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Phrasal Verbs

rat on somebody

(North American English also rat somebody out (to somebody))
 
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(informal, disapproving) to tell somebody in authority about something wrong that somebody else has done Where I come from, you don't rat on your friends.
(British English, informal) to not do something that you have agreed or promised to do synonym renege The government is accused of ratting on its promises to the unemployed.
See the Oxford Advanced American Dictionary entry: rat on