slag off

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slag off (third-person singular simple present slags off, present participle slagging off, simple past and past participle slagged off)

1. (Britain, Ireland, Australia, slang, transitive, idiomatic) To talk insultingly to or about someone or something.
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.