Source : Webster's Revised Unabridged Dictionary (1913)

Wild-cat \Wild"-cat`\, a.
   1. Unsound; worthless; irresponsible; unsafe; -- said to have
      been originally applied to the notes of an insolvent bank
      in Michigan upon which there was the figure of a panther.

   2. (Railroad) Running without control; running along the line
      without a train; as, a wild-cat locomotive.