1. To criticize or upbraid someone in a harsh, insulting, and abusive manner. A noun or pronoun can be used between "slag" and "off." Primarily heard in UK, Ireland. I wish the boss would offer some constructive criticism instead of just slagging us when something goes wrong. Each night, I can hear the woman next door slagging off her husband. The boss slagged my idea off in front of everyone at the meeting.
2. To tease, mock, or ridicule someone. A noun or pronoun can be used between "slag" and "off." Primarily heard in UK, Ireland. Ah, don't take everything so personally, I'm only slagging you! It took me a while to get used to the way Sarah's family slags each other off all the time. I had a feeling the other kids would slag off my clothes, but I didn't care.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.