knock on wood = to rap on something made of wood. (Said as a wish for good luck. Usually a phrase attached to another statement. Sometime said while knocking or rapping on real wood.) I think I am well at last—knock on wood. I knock on wood when I wish something were true.

"Knock on wood" = "Knock wood for luck - we need it." Old superstition was that knocking (or touching) wood was good luck. "We can have a pretty good picnic if it doesn't rain, knock on wood." [If we're lucky, it won't rain on our picnic - knock wood just for luck.]

The phrase "knock on wood" is used as a saying of warding off bad luck or wishing for good luck to continue. The phrase is often used at the end of a sentence in which someone is telling about their good fortune. Examples: My car has been running well this year. Knock on wood! My wife and children are happy and healthy. Knock on wood! I do not owe any money to creditors. Knock on wood! Hope this helps! :-)