unfasten

[uhn-fas-uh n, -fah-suh n] /ʌnˈfæs ən, -ˈfɑ sən/
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.
Origin
1175-1225; Middle English unfastnen. See un-2, fasten
British Dictionary definitions for unfastened

unfasten

/ʌnˈfɑːsən/
verb
1.
to undo, untie, or open or become undone, untied, or opened
Word Origin and History for unfastened

unfasten

v.

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