Definition of black liquor in English:

black liquor

noun

  • 1rare Any of various black liquids used in dyeing; specifically a solution of lead acetate used as a mordant; also called iron liquor; (now rare).

  • 2A liquid residue containing lignin compounds and inorganic chemicals, formed when pulpwood is heated in alkaline solution in the kraft papermaking process.

Origin

Early 18th century; earliest use found in Whole Art Dying.