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]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "unstop":
   bare, clear, deobstruct, disclose, expose, free, manifest, ope,
   patefy, reveal, unblock, uncase, unclench, unclog, unclutch,
   uncork, uncover, uncurtain, undo, undrape, unfold, unfoul, unlatch,
   unlock, unplug, unroll, unseal, unsheathe, unshut, unveil,
   unwrap