The Collaborative International Dictionary of English v.0.48:

Jigger \Jig"ger\, n. [See Jig, n. & v.]
   1. One who, or that which, jigs; specifically, a miner who
      sorts or cleans ore by the process of jigging; also, the
      sieve used in jigging.
      [1913 Webster]

   2. (Pottery)
      (a) A horizontal table carrying a revolving mold, on which
          earthen vessels are shaped by rapid motion; a potter's
          wheel.
      (b) A template or tool by which vessels are shaped on a
          potter's wheel.
          [1913 Webster]

   3. (Naut.)
      (a) A light tackle, consisting of a double and single
          block and the fall, used for various purposes, as to
          increase the purchase on a topsail sheet in hauling it
          home; the watch tackle. --Totten.
      (b) A small fishing vessel, rigged like a yawl. [New Eng.]
      (c) A supplementary sail. See Dandy, n., 2
      (b) .
          [1913 Webster]

   4. A pendulum rolling machine for slicking or graining
      leather; same as Jack, 4
      (i) .
          [1913 Webster]

   5. A small glass or measuring vessel holding 11/2 ounces (45
      ml), used mostly for measuring liquor or drinking whiskey;
      also, the quantity of liquid held in a jigger.
      [PJC]

   6. A thingamajig. [Colloq.]
      [PJC]

   Jigger mast. (Naut.)
      (a) The after mast of a four-masted vessel.
      (b) The small mast set at the stern of a yawl-rigged boat.
          [1913 Webster]