Is Slack-Jawed an Idiom?

(of a person) With the mouth in an open position and the jaw hanging loosely, especially as indicating bewilderment or astonishment. (idiomatic, of a person) Unsophisticated or unthinking; dimwitted in appearance. (idiomatic, dated, of a person) Overly talkative; indiscreet.

Why are some people slack jawed?

There is a reason

No medical folk I checked with believed today's popular slack-jawed look is any more common among oldsters than bewildered youth. But old or young, the most likely cause is chronic mouth breathing, maybe caused by nasal obstruction or weak diaphragm or abdominal muscles.

What type of word is slack?

intransitive verb. 1 : to be or become slack. 2 : to shirk or evade work or duty. transitive verb. 1a : to be slack or negligent in performing or doing.