open (one's) big mouth(redirected from open their big mouth)
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open (one's) big mouth
To say something that is meant to be a secret; to say something in an indiscreet, noisy, or boorish manner. Why did you have to open your big mouth about my crush on Tommy? Why couldn't you just keep it between us? We would have gotten away with the plan if she hadn't opened her big mouth!
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.