It turns out “touch wood” or “knock on wood” has been used across many different countries so it’s hard to say who can lay claim to its first usage. There is at least some sound grounding for its origin in German folklore where spirits were thought to live in wood and by knocking on the wood you were able to gain their protection.
While there is no real way for us to quantify why Americans appropriated the phrase as “knock on wood” from “touch wood” we at least have some data to show the timeline for it.
Looking at the numbers, it seems “knock on wood” only started to appear in the lexicon around 1900 both in America and England, whereas “touch wood” had been used in English as far back as 1750. It seems “knock on wood” gained some usage in British English though it is far out used by “touch wood”. Meanwhile “knock on wood” outpaced “touch wood” in American English around 1950.
It would be interesting to learn whether “knock on wood” originally sprouted in England and transplanted over or if it was American adaptation that eventually transplanted the other way. Either way no matter which you use I’d like to think the magic of phrase works.
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