- 1
To open something that was held closed — a button, a belt, a zip, a rope, a lock, a clip. means to join two things so they stay shut; un- undoes it. So you your seatbelt at the start of a flight and unfasten it at the end. The word is calm and careful. It is what you say when the opening is deliberate and gentle — unfastening a necklace, unfastening a child's coat. If something is ripped open, you would not use this word. In everyday speech, English speakers usually reach for a more specific verb: unbutton a shirt, unzip a bag, a rope, take off a belt, or just "undo" — undo is the most common of all in British English, and covers nearly everything unfasten covers. Unfasten survives strongly in one place: on planes and in cars, in the announcement "please keep your seatbelts ," and so "unfasten your seatbelt" sounds completely natural to everyone. Elsewhere it feels a little careful or written.
解開、鬆開（扣子、安全帶、繩子等）
組成un- (undo the action) + fasten (to join or fix so it stays shut). Note that this un- is not "not" — it means "reverse it," the same un- as in unlock and untie.
原始詞典資料2 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。
unfastenverb/ʌnˈfɑːs(ə)n/
- 1
To detach from any connecting agency or link; to disconnect.
- He doth unfasten so and shake a friend.
- 2
To come unloosed or untied.
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。