告密者

tell-tale / informer (esp. to police) / whistleblower / grass

(after a verb or adjective) one who (is) ... / (after a noun) person involved in ... / -er / -ist / (used after a number or 後|后[hou4] or 前[qian2] to refer to sth mentioned previously) / (used after a term, to mark a pause before defining the term) / (old) (used at the end of a command) / (old) this

to say / to tell / to announce / to report / to denounce / to file a lawsuit / to sue

secret / confidential / close / thick / dense

to inform against sb