When you unfasten something, you loosen or open it. You should wait to unfasten your seatbelt until the car has completely stopped.

To fasten is to hook, close, or latch something, and to unfasten it is to undo it. You might unfasten your dog's leash so she can run free on the beach or unfasten the snaps and buttons on your jacket so you can take it off. You can also unfasten your ponytail, letting your hair down, or unfasten the latch on a gate. The Old English root word is fæstnian, "make fast or firm."

Definitions of unfasten
  1. verb
    cause to become undone
    unfasten your belt”
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    fasten, fix, secure
    cause to be firmly attached
    remove the chains from
    open the zipper of
    unfasten, as a sail, from a spar or a stay
    type of:
    cancel, annul, or reverse an action or its effect
  2. verb
    become undone or untied
    “The shoelaces unfastened
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    become fixed or fastened
    type of:
    open, open up
    become open
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