The term "dross" is used to describe waste material which must be removed and processed separately or discarded in a safe place. It may also be dangerous, poisonous, or impure, depending on the context of the usage. This word from from an Old German word meaning "dregs," and it was originally used in reference to manufacturing processes.
In the original sense, people used "dross" to refer to impurities which arise during the smelting process. As metal is melted, it oxidizes, forming a layer of scum and impurities which float to the surface as the metal heats up. This dross can be skimmed from the metal so that the refined product will be more pure, ensuring a reliable performance when it is cast or worked. With some metals, the amount of dross generated can be very high, as much as 50% of the metal, and this can drive the cost of the metal up significantly.
In addition to containing the products of oxidation, the dross can contain a variety of unwanted impurities which have gotten into the metal. In some cases, these impurities can be dangerous in addition to unwanted, explaining the occasional usage of the word to describe something which is potentially toxic. This type of dross needs to be handled and processed with special care so that it does not pose a risk to human health, and it can sometimes cause problems in refineries and forges, as it may offgas dangerously when heated.
Over time, people came to use the term more generally to refer to impurities. For example, a city might say that it processes its water to remove dross before delivering it to the end consumer, meaning that it is filtered to pull out potentially dangerous impurities. People may also the term when describing other waste materials which emerge during other types of refining processes.
People can also talk about dross metaphorically. For example, someone might say "I was able to get more work done once I cleared my mind of mental dross." In this case, it is not a physical impurity which can be skimmed away, but a collection of unwanted and distracting thoughts which someone manages to clear so that he or she can focus. The elimination of mental dross is a key goal of disciplines such as meditation and guided imagery, and people are sometimes encouraged to discard the mental form of waste materials so that they can focus their thoughts and efforts more effectively.