Apart from other more obvious meanings, Slag is a waist product produced inside a Blast Furnace. It is produced when Limestone is used to remove impurities in Iron. This can be difficult to dispose of and may form slag heaps when dumped in the environment. These are large mounds of thrown away slag.
Citizen: I hate all this waist slag polluting the local environment.
Council member: Don't worry; we are currently working on removing all of the slag heaps.