In using lead-tin solder in large quantities, as in repairing a large galvanized iron
pail, the molten solder oxidizes on to forming a film of dross that must be skimmed
off, as it can interfere with solder adhering to the work.
In working on printed circuit boards with a very small soldering iron and rosin core
solder, the iron tip has to be wiped immediately before being used for the same reason.
So the inert powdery oxides of the solder is called dross.
As a metaphor the term can be used to mean something worthless as dregs in something
of value.