The Collaborative International Dictionary of English v.0.48:

Unstock \Un*stock"\, v. t. [1st pref. un- + stock.]
   [1913 Webster]
   1. To deprive of a stock; to remove the stock from; to loose
      from that which fixes, or holds fast.
      [1913 Webster]

   2. To remove from the stocks, as a ship.
      [1913 Webster]