Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

unpin
     v : remove the pins from; unfasten the pins of [ant: {pin}]
     [also: {unpinning}, {unpinned}]