slack-jawed

adjective3

1. •
   With the mouth in an open position and the jaw hanging loosely, especially as indicating bewilderment or astonishment.

   • For one frightful moment there was no sound; even the men's breathing was hushed, and they sat slack-jawed, stunned, half-minded to believe this some hideous, incredible jest. — 1911
2. •
   Unsophisticated or unthinking; dimwitted in appearance.

   • She brought to the stage the barefoot, slack-jawed frontiersmen found on postcards in every convenience store and tourist trap in the Ozarks. — 2002
3. •
   (old-fashioned) Overly talkative; indiscreet.