Idiom: To knock it off.
Example 1: My neighbor is a guitar player (but he's not very good). I try to study at home in the evenings but I can't concentrate with the "music" coming through the walls. Some day, I'm going to go over there and tell him to knock it off. I can't take it anymore!
Cindy: Hey Penny, you look tired. Are you feeling OK?
Penny: Hey.. I didn't sleep much last night. I got a new video game and I've been playing it all day and night for the last 2 days. I only take a break when I get hungry, but I just eat something simple like potato chips so I can get back to playing faster.
Cindy: Penny! You'd better knock it off before you get sick!
Meaning: "To knock it off" (phrasal verb) means to stop doing something annoying or bad. This expression is usually used as a command or imperative.
...I'm going to go over there and tell him to stop it.
...You'd better stop it before you get sick!
In each case, it is the activity that should be stopped.
You can learn more idioms like this in LSI's Intermediate and Advanced Conversation Programs.
Please see www.languagesystems.edu for more information.