slag off

verb2

1. •
   (UK, slang) To talk insultingly to or about someone or something; usually one who is not present.

   • I am forever slagging off some poor harmless dude or pitiful demented twitcher[.] — 1983
2. •
   To purify by melting and removing slag.

   • There is a lot of copper oxide, but I can remove that by melting everything and slagging it off.