mouth on (one)
To share or reveal compromising information about one to a figure or body of authority. If you mouth on them to the cops, they'll definitely try to have you killed! Why are you always mouthing on me like that! You're such a tattletale!
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
mouth on someone
to inform the authorities on someone; to tattle on someone. Max mouthed on his accomplice and got him arrested. You had better not mouth on me!
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.