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.