inform on (someone)
To share or reveal compromising information about someone, usually to the authorities. If you inform on him to the cops, he'll definitely send some of his goons out after you. Yeah, I know about your shady deals, but I didn't inform on you, I swear!
inform someone on someone
to tattle (on someone) (to someone). I will inform the teacher on you! Billy informed his mother on Bobby.
inform on someone
to tell the authorities about someone; to tattle on someone. I am going to have to inform on you. Liz informed on Ken to their mother.
inform on
or inform againstv. To disclose confidential or incriminating evidence about someone to an authority: The FBI agent informed on the drug dealers. You can't force me to inform against my own family.