Wiktionary
slag off
vb. (context British Irish Australia slang transitive idiomatic English) To talk insultingly to or about someone or something.
Usage examples of "slag off".
In Kingdom by the Sea, Paul Theroux gives the impression that you can walk it in an easy lope and still have time for a cream tea and to slag off the locals, but I trust he had better weather than I.
I'm not some embittered hash-slinger out to slag off my more successful peers (though I will when the opportunity presents itself).