Also, touch wood. Express a wish that something will or will not occur, as in This last round of treatment should have cured her, knock on wood. This expression alludes to an ancient superstition that literally knocking on or touching wood will ward off evil spirits. [c. 1900]

Some people say, “Knock on wood,” and then knock on something made of wood for good luck, when they have made a remark that has been true up to that point and they want it to continue to be true: “I've never had an accident yet, knock on wood.”