> Knocking on wood was about knocking on unpainted wood, so that you have actual direct physical contact with wood.

How do you know this? For example, there is a wikipedia page section entitled "Origin of use in the English language"...

https://en.wikipedia.org/wiki/Knocking_on_wood#Origin_of_use...

but the contents of that section make it extremely clear that no relevant facts are known to wikipedia.

N >= 2