slack-jawed

slack-jawed (comparative more slack-jawed, superlative most slack-jawed)

1. (of a person) With the mouth in an open position and the jaw hanging loosely, especially as indicating bewilderment or astonishment.
   • 1911, Rex Ellingwood Beach, The Ne'er-Do-Well, ch. 26:

     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.

2. (idiomatic, of a person) Unsophisticated or unthinking; dimwitted in appearance.
   • 2002, Brooks Blevins, Hill Folks: A History of Arkansas Ozarkers and their Image, →ISBN, p. 262:

     She brought to the stage the barefoot, slack-jawed frontiersmen found on postcards in every convenience store and tourist trap in the Ozarks.

   • 2005, Michelle Cottle, "My Roving Barcalounger," Time, 24 July:

     But driving a car—particularly the supersize models—really should demand more concentration than, say, slouching slack-jawed in front of the wide screen in your den.

3. (idiomatic, dated, of a person) Overly talkative; indiscreet.
   • 1917, P. G. Wodehouse, "Wilton's Holiday" in The Man with Two Left Feet and Other Stories:

     Spencer was one of those slack-jawed youths who are constitutionally incapable of preserving a secret.

• (with the mouth open and the jaw hanging loosely): dazzled, dumbfounded, open-mouthed, stunned
• (unsophisticated; dimwitted in appearance): dopey, stupid, stupefied
• (overly talkative): big-mouthed

• mouth breather