Definition of blue billy in US English:

blue billy

noun

historical
  • 1Impure lime containing blue iron compounds, especially as a waste product of coal gas manufacture; liquid containing this.

  • 2Copper-containing iron pyrites residue from the processing of copper ore.

Origin

Mid 19th century; earliest use found in Repertory Patent Inventions. Apparently from blue + the male forename Billy, pet-form of William.