Where did the term 'knock on wood' come from?

Question

I was just wondering where that expression came from and why everyone is so fanatical about it. Anyone who can answer will be much appreciated.

Answer 1

I've heard two theories, and I suspect one was designed to replace the other. (1) It is the Druid's way to call up the spirits inside the tree, and (2) it is the Christian's way to touch the cross on which Jesus was crucified.

You pays your money and you takes your pick.

Answer 2

It comes from people invoking spirits that live in trees, in the old times.