Definition of jack sinker in English:

jack sinker

noun

historical, rare
  • (In a stocking frame or knitting machine) each of a series of thin metal plates attached to the jacks and suspended vertically between the needles, used in conjunction with the lead-sinkers to form loops upon the thread.

Origin

Mid 18th century; earliest use found in George Charles Deering (?1695–1749), botanist.