In this context, "slag" refers to the waste material that is produced during the smelting process. Smelting is a method of extracting metal from its ore by heating and melting it to separate the desired metal from impurities. The waste material that is left behind after the metal has been extracted is called slag.
Note: The provided meaning of the answer is for the latest New York Times crossword puzzle clue: Smeltery waste. Explanations are generated by AI and may be inaccurate.