To absent oneself or leave early from school or work when one would normally be required to be there; to play truant. Primarily heard in UK. I was so restless and bored at work that I decided to just sag off after lunch without telling anyone. Hey, Jim and I are planning on sagging off from school on Friday, do you want to come with us? That's the last time you sag off class, mister! From now on, I'm dropping you off to school every morning!