unfasten
1. [ verb ] cause to become undone
Examples:

"unfasten your belt"

Related terms: fasten undo unzip unbend unchain
2. [ verb ] become undone or untied
Examples:

"The shoelaces unfastened"

Related terms: fasten open untying
Similar spelling:   unfastener
  unfastened
  unfastening
  unbiased
  unfaceted
  unbiassed
  unpassable
  unfeasible