1. Someone prone to talking a lot or at length. Good luck getting out of here—Aunt Louise is a real flap-jaw and could easily blather to you for an hour.
2. Someone who too readily shares information, especially that which is private or confidential. Some flap-jaw has been talking to the press about our new product. Needless to say, if I find out who it is, they will be fired on the spot. If you've got a secret, don't tell it to a flap-jaw like Marty!
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.