Source : Webster's Revised Unabridged Dictionary (1913)

Unfasten \Un*fas"ten\, v. t. [1st pref. un- + fasten.]
   To loose; to unfix; to unbind; to untie.

Source : WordNet®

unfasten
     v 1: cause to become undone; "unfasten your belt" [ant: {fasten}]
     2: become undone or untied; "The shoelaces unfastened" [ant: {fasten}]