Source : Webster's Revised Unabridged Dictionary (1913)

   Note: Jack is used adjectively in various senses. It
         sometimes designates something cut short or diminished
         in size; as, a jack timber; a jack rafter; a jack arch,
         etc.

   {Jack arch}, an arch of the thickness of one brick.

   {Jack back} (Brewing & Malt Vinegar Manuf.), a cistern which
      receives the wort. See under 1st {Back}.

   {Jack block} (Naut.), a block fixed in the topgallant or
      royal rigging, used for raising and lowering light masts
      and spars.

   {Jack boots}, boots reaching above the knee; -- worn in the
      17 century by soldiers; afterwards by fishermen, etc.

Back \Back\, n. [F. bac: cf. Arm. bak tray, bowl.]
   1. A large shallow vat; a cistern, tub, or trough, used by
      brewers, distillers, dyers, picklers, gluemakers, and
      others, for mixing or cooling wort, holding water, hot
      glue, etc.

   {Hop back}, {Jack back}, the cistern which receives the
      infusion of malt and hops from the copper.

   {Wash back}, a vat in which distillers ferment the wort to
      form wash.

   {Water back}, a cistern to hold a supply of water; esp. a
      small cistern at the back of a stove, or a group of pipes
      set in the fire box of a stove or furnace, through which
      water circulates and is heated.

   2. A ferryboat. See {Bac}, 1.