To knock means to hit or strike something, usually to make a sound that gets someone's attention. When you knock on a door, you rap your knuckles against it to let people inside know you're there. You might knock on wood for good luck, or knock over a glass of milk by accident at dinner.

The word also describes the sound itself: you hear a knock at the door when someone arrives. A car engine makes a knocking sound when something's wrong with it. In sports, a boxer might knock out an opponent with a powerful punch, while a baseball player tries to knock the ball over the fence.

Knock can also mean to criticize or put something down. If your friend knocks your favorite movie, they're saying negative things about it. When people knock an idea, they're pointing out its flaws or dismissing it.

You'll hear knock in many phrases: knock it off means stop doing something annoying, while knock yourself out means go ahead and do whatever you want. If something knocks your socks off, it amazes you. And when opportunity knocks, it means a good chance has appeared, like the expression “opportunity knocks but once.”