The Collaborative International Dictionary of English v.0.48:

Unfasten \Un*fas"ten\, v. t. [1st pref. un- + fasten.]
   To loose; to unfix; to unbind; to untie.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "unfasten":
   cast off, detach, disengage, doff, free, liberate, loose, loosen,
   release, remove, take off, unbar, unbind, unbolt, unbuckle,
   unbutton, unchain, unclasp, undo, unfetter, unglue, unlace,
   unlatch, unleash, unlock, unloose, unloosen, unstick, unstrap,
   untie, weigh anchor