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:
2012年12月，党的十八大闭幕不久，习近平总书记来到地处太行山深处的河北阜平县，进村入户看真贫。
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