DuckCorp Dico

Found one definition

1.                 From The Collaborative International Dictionary of English v.0.48:
                   
   
   
                   
                     
   Unpin \Un*pin"\, v. t. [1st pref. un- + pin.]
      To loose from pins; to remove the pins from; to unfasten; as,
      to unpin a frock; to unpin a frame.
      [1913 Webster]