From Longman Dictionary of Contemporary Englishunbuckleun‧buck‧le /ʌnˈbʌkəl/ verb [transitive] FASTEN/DO UPto unfasten the buckle on a belt, a shoe etc→ See Verb table
Examples from the Corpus
unbuckle• Finally, he decides to unbuckle his belt, loosen his pants, sit back and relax.• The gunman managed to unbuckle his safety belt and struggled to push open the passenger door.• That day the dashing young man unbuckled his sword and never wore one again.• Her hands slid under his shirt, unbuckling, unzipping, thrusting on downwards. 