The Collaborative International Dictionary of English v.0.48:

merkin \mer"kin\, n.
   1. Originally, a wig; afterwards, a mop for cleaning cannon.
      [1913 Webster]

   2. A patch of false hair, or something resembling hair, worn
      on a string over the female pudenda; -- used by
      stripteasers.
      [PJC]

   3. The hair on the female pubic area. [slang]
      [PJC] Merl