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If someone is slack-jawed, their mouth is a little open because the jaw is hanging loose. The usual picture is surprise: you see something huge or strange and your mouth falls open. It can also paint a person as slow or not thinking, which is . You meet it in stories and film writing more than in careful speech. Informal. Slack here means loose, not pulled tight. Though actually, people sometimes use it with a laugh, for a moment of stunned silence, and sometimes as a quiet insult. Listen to the tone before you copy it.
下巴鬆開、目瞪口呆的（有時帶「呆呆的」意思，不太客氣）
原始詞典資料3 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
slack-jawedadj
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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.
- The Pudgelys and the Pinkhearts and the Roundasses are all staring at her, slackjawed.
- 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.
- 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
datedOverly talkative; indiscreet.
- As a matter of fact, it was by trying to lean on it that Spencer Clay got hold of the facts of the case; and when young Clay got hold of anything, Marois Bay at large had it hot and fresh a few hours later; for Spencer was one of those slack-jawed youths who are constitutionally incapable of preserving a secret.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。