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:
平日里，纳泽习惯刷卡或现金支付。来中国前，他曾担心支付不便，来之后，才发现担心多余。
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The funnel-neck jacket is boxy, generously cut and comes with a permanently popped collar, between 9cm and 14cm high running from clavicle to nose; high enough to cover your mouth, low enough to see out – just.