flap (one's) gums
To chatter or blather. Quit flapping your gums—I need some quiet so I can think! Whenever Charlie starts to flap his gums, I can't get in a word!
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
flap one's gumsand flap one's jaws
Rur. to talk aimlessly. They're still out on the porch, flapping their gums. Well, I can't sit here flapping my jaws all day. Gotta get back to work.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.