1 general:: Phrase(s): rat on someone Inf. to report someone’s bad behavior to someone in authority; to tattle on someone. • John ratted on me, and I got in trouble. • If he rats on me, I’ll hit him!
McGrawhill's American Idioms And Phrasal Verbs
2 general:: Betray a comrade by giving information, as in He ratted on his best friend to the police. [Slang; early 1900s]
American Heritage Idioms