Knock On Wood's Catholic Origens
...has Catholic origens, rather than superstition alone.
I read, in the book Why Do Catholics Eat Fish on Friday?, the following...
"The Cross is remembered not only by virtue of its shape or form, as with the sign of the cross, but by virtue of its material. The instrument of Christ's death was often the object of reflection by the Church Fathers, Christianity's illustrious early thinkers and bishops, who saw the wooden cross as the new tree of eternal life that rescues us from slavery to which Adam and eve's sin with the tree of the knowlege of good and evil bound us. The supersition of knocking on wood is thus a way of involving the power of the cross in the face of danger or misfortune. In some areas it is even said that one is supposed to knock on wood three times, one for each Person of the Trinity*."
* Religious Superstition Through the Ages by Don Lewis (p.35)