Why “dross” is a great word
The worthless scum of impurity that rises to the surface of molten metal during smelting, from the Old English drōs ("dregs, sediment"), from Proto-Germanic *drōhs ("dregs, sediment"), ultimately from Proto-Indo-European *dʰrā́ks ("sediment, yeast"). Unlike "slag," which is the separated, often vitreous by-product that can find secondary use, or "dregs," the final, bitter residue of a liquid, dross is the specific, oxidized filth of the furnace. It is the metallic froth skimmed and discarded, the tarnish that mars pure shine, the inherent corruption brought to light only by the purifying fire—a reminder that value is often wrested from a fundament of base and floating waste.
Etymology
From Middle English drosse, dros, from Old English drōs, from Proto-Germanic *drōhs (“dregs, sediment”).
Also compare Old English drōsna, drōsne (“a ground, sediment, lees, dregs, dirt, ear wax”), from Proto-Germanic *drōhsnǭ, *drōhsnō (“dregs, sediment”), derived from *drōhs. Alternatively, this may be from *dragjō + *-snō (“yeast, sediment”; compare *dragjō (“yeast”)), as if from *drēcg + -sn.
Ultimately from Proto-Indo-European *dʰrā́ks (“sediment, yeast”).
Cognate with Scots dros, drose, drosse (“small particles, fragments, dross”), Middle Dutch droes (“dregs”), Dutch droesem (“dregs”), German Drusen (“lees, dregs”), Latin fracēs (“grounds or dregs of oil”). Related also to drast, dregs.