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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（五）从建筑物或者其他高空抛掷物品，有危害他人人身安全、公私财产安全或者公共安全危险的。。关于这个话题，safew官方版本下载提供了深入分析
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