To rat out means to reveal incriminating or embarrassing information about someone, especially to a person in authority.
To tell someone in authority about the wrong or illegal actions of another.
- To tell on someone.
1. Johnny ratted out the group of vandals to the principal.
2. Karen ratted out the other girls who were smoking in the washroom.
3. He ratted me out to the police.
4. You’d better not rat me out.
betrayer, blabbermouth, canary, double-crosser, fink, informant, play false, rat, sell one’s soul, sneak, snitch, source, squealer, stab in the back, stoolie, tattler, tattletale, turncoat, whistle-blower