RhymeZone: slack-jawed definitions

Definitions of slack-jawed:

adjective: With the mouth in an open position and the jaw hanging loosely, especially as indicating bewilderment or astonishment.
adjective: (by extension) Unsophisticated or unthinking; dimwitted in appearance.
adjective: (dated) Overly talkative; indiscreet.

(Definitions from Wiktionary)