† slackstoneObs.rare. [ad. G. schlackstein(seequot. in Grimm).] A form of slag. 1683Pettus Fleta Minor i. xiv. 41 How a Slackstone or Copper-stone is to be made.Ibid., Slackstones (as the Philosophers do judg) are Sulphur and Arsnick mingled with a subtil Earth.