The Collaborative International Dictionary of English v.0.48:

Unbuckle \Un*buc"kle\, v. t. [1st pref. un- + buckle.]
   To loose the buckles of; to unfasten; as, to unbuckle a shoe.
   "Unbuckle anon thy purse." --Chaucer.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unbuckle
    v 1: undo the buckle of; "Unbuckle your seat belt" [ant:
         buckle, clasp]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "unbuckle":
   cast off, detach, disengage, doff, free, liberate, loose, loosen,
   release, remove, take off, unbar, unbind, unbolt, unbridle,
   unbutton, uncage, unchain, unclasp, undo, unfasten, unfetter,
   ungag, unglue, unhandcuff, unhobble, unlace, unlash, unlatch,
   unleash, unlock, unloose, unloosen, unmanacle, unmuzzle, unpen,
   unpinion, unshackle, unstick, unstrap, untether, untie, untruss,
   unyoke, weigh anchor