1. To criticise or upbraid someone in a harsh, insulting, and abusive manner. 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.

2. To tease, mock, or ridicule someone. 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.