1. To bump or strike someone on the head, either accidentally or intentionally. That book knocked me on the head when it fell off the shelf. Quit messing with me or I'll knock you on the head!

2. To promptly prevent something, especially an idea, suggestion, or plan, from being developed, proliferated, or carried out. Primarily heard in UK. We need to knock this notion of forming a union on the head before it gets too far. He's been trying to knock the rumours on the head, but by now nearly everyone has heard them.

3. To stop (doing) an activity or behavior. Primarily heard in UK. You need to knock that smoking on the head before the baby arrives. I briefly got into computer programming, but I knocked it on the head when things got more complicated.