verb (v)

• cause to become undone(verb.contact)

  Example:
  Unfasten your belt.
  

  Antonym:
  fasten, fix, secure
  

  source: wordnet30
• become undone or untied(verb.contact)

  Example:
  The shoelaces unfastened.
  

  Antonym:
  fasten
  

  source: wordnet30
• To loose; to unfix; to unbind; to untie.(verb)

  source: webster1913