"knock on wood"
When Saint Helen, mother of Emperor Constantine the Great, visiting the Holy Land and seeking the True Cross upon which ***** was crucified, found the Precious and Life-Giving Cross, the Cross was venerated by all the faithful in many public processions. Many faithful would come to touch the Holy Cross for blesssing and healing. It was customary to touch the Wood of Life three times (as a confession of faith in the Holy Trinity). This act of touching the True Cross became the earliest recorded histories of "knocking on wood." This history, alas, has nothing to do with the Rosary since the Rosary is a much later innovation. Whenever the Holy Cross was put forth for public veneration, touching it, or as English translations render it "knocking," became common liturgical practice. Once the Holy Cross was transferred to Constantinople and placed in the Great Church of the Holy Wisdom (aka Saint Sophia), Christians continued this piety by touching or knocking on any cross or crucifix (wood was the medium of the day) for blessings and healings. This ancient tradition has been with us for over 1,600 years and has been a pious tradition to this day where people tend to touch anything made of wood ... but all interpretations of this behavior point back directly to Jerusalem in the 4th century AD and the True Cross. No doubt, wooden prayer beads may have become a customary item for similar veneration, but historically much later.