It means "stop doing something" but directed at someone else. If someone keeps playing loud music, you can ask them to "knock it off." I hope that example helps.

A commanding phrase meaning "Stop it!" I say this to my child when she is making a mess or being noisy. "Hey, knock it off!"

I agree with Sara, and to add an example : My wife was so slow in getting ready that day, that we missed the train. So, I kept moaning and blaming her for that. After I told her for the third time that it was her fault and that she should have taken less time to get ready, she said: "Enough now...knock it off !"

That phrase is used when someone wants someone else to stop doing something. It's informal and usually used when someone is mad or irritated.

Thank you my friends for your help!