dross

Short definition from Wiktionary

Noun

1. 1
   Waste or impure matter: Residue that forms as a scum on the surface of molten metal from oxidation.
2. 2
   Waste or impure matter: The impurities in metal.
3. 3
   Waste or impure matter: A waste product from working with metal.
4. 4
   Worthless or trivial matter.
5. 5
   Residual raw opium left in an opium pipe which can be recycled for further sale or use.

Verb

1. 1
   To remove dross from.