Any substance, whether natural or synthetic, used as a coloring agent or dye to color fabrics, fibers, or other materials.
From dye plus stuff (matter or material), a compound that emerged in textile terminology to denote the physical material basis of dyes. The term standardized in English during the Industrial Revolution as dye chemistry became more scientific.
Dyestuff became a technical term that lets people distinguish between the chemical material itself and the process of dyeing—it's what gets mixed into the vat, while dyeing is what happens to the fabric.
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