What I found is that written evidence for knocking on wood (and for the history of gesture as a whole) is remarkably sparse. In the Penguin Guide to Superstitions, folklorist Steve Roud offers a skeptical take on the provenance of knocking on wood due to the near total absence of written records about it:
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In 1992, in a small shop in British Columbia, a sign maker named Blair Gran stared at a wall full of half-finished jobs and felt something click. Sign-making was treated like a commodity — orders in, banners out — but as thousands of signs came through his shop, he couldn’t help but notice the difference between the good ones and the bad ones. He could see that every sign that left his shop was either helping a business get noticed, or letting it disappear in plain sight.，详情可参考safew官方下载
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