Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

unbuckle
     v : undo the buckle of; "Unbuckle your seat belt" [ant: {buckle}]