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1. To speak without discretion; to speak too loudly or freely, especially about sensitive topics or information. We would have gotten away with our plan if your dumb cousin hadn't started mouthing off all over town.
2. To make disrespectful comments; to sass someone. You're going to get detention again if you keep mouthing off to your teacher.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
to speak out of turn; to backtalk. If you mouth off, I will ground you for three weeks. Don't mouth off at me, buster!
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
1. Complain or express one's opinions loudly and indiscreetly, as in She was always mouthing off about the other members. [Slang; 1960s]
2. Speak impudently, talk back, as in He got in trouble by mouthing off to his teacher. [Slang; 1960s]
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
To express one's opinions or complaints in a loud, indiscreet manner: Quit mouthing off about what you think is wrong, and go do something about it! Don't mouth off to your parents like that!
The American Heritage® Dictionary of Phrasal Verbs. Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
in. to give (someone) back talk. If you mouth off, I will ground you for three weeks.
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.