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1. To speak (about something) 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 lipping off all over town. Don't go lipping off about our plans like that. You never know who might be listening in!
2. To speak in a disrespectful, insolent, or impudent manner. You're going to get detention again if you keep lipping off to your teacher. If you ever lip off like that again, I'll tan your hide!
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.