slag off

verb

1. 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.”

2. 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[.]”

   “There’s pressure to be a brand. I’m even locked out of my social media because I just get drunk and slag people off.”