2) The tiny particles of metal which a grindstone throws off when knives and tools are given a cutting edge. It was apparently used as a black dye.
1565 no person ... shall dye or cause to be dyed black, any cap, with bark or swarf
1568 In swarffe xxviijs Ireon in the shope, Kendal. The Sheffield cutlers had a protective shield which caught moisture and the stone and metal particles from the grindstone: in 1714, Henry Shaw, a Wadsley grinder called Henry Shaw had 2 swarfing boards at his grinding wheel.