Posted by Jim McCracken on January 01, 2006
Knock on wood.
Origin given as:
May be the association that wood and trees have with good spirits in mythology, or with the Christian cross.
The version I heard was that churches in medieval times were places of sanctuary but disputes arose as to when sanctuary was reached, entry onto church grounds, actual entry into the church etc. It was then supposedly decided that sanctuary was reached when one touched the door of the church, thus touch (or knock on) wood.
I obviously don't know if it's true, but it sounds plausible.