unfasten

verb (used with object)

1. to release from or as from fastenings; detach.
2. to undo or open (something fastened).

verb (used without object)

3. to become unfastened.

verb

1. to undo, untie, or open or become undone, untied, or opened

v.early 13c., from un- (2) + fasten. Related: Unfastened; unfastening.