1
general::
Phrase(s): 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.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
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]
American Heritage Idioms