The Collaborative International Dictionary of English v.0.48:

Unstop \Un*stop"\, v. t. [1st pref. un- + stop.]
   [1913 Webster]
   1. To take the stopple or stopper from; as, to unstop a
      bottle or a cask.
      [1913 Webster]

   2. To free from any obstruction; to open.
      [1913 Webster]