Slag off
Definition 1 of 1
Pro Tip 1/3British Slang
Primarily used in British English, might not be understood globally. 
She might find it confusing if you slag off her favourite singer; she's not familiar with British slang.
Pro Tip 2/3Colloquial Critique
Use 'slag off' in informal situations or among friends, not in professional settings. 
I overheard them slagging off the new policy in the break room.
Pro Tip 3/3Negative Shade
Carries a negative tone, indicating strong disapproval or disdain. 
It's not fair to slag off someone who's not here to defend themselves.