knock (someone or something) on the head
Primarily heard in UK.
1. To promptly prevent something, especially an idea, suggestion, or plan, from being developed, proliferated, or carried out. 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.
2. To stop (doing) an activity or behavior. 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.
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