In the United States, we say “knock wood or knock on wood” (in the U.K., it’s “touch wood”) in a variety of situations, like after mentioning something we hope will happen, or while discussing something good that we want to remain in a positive place. It’s a means of averting misfortune, making sure we don’t “tempt fate.” Some explanations for the practice mention a Celtic or otherwise pagan association with tree spirits, the idea being that knocking on wood (particularly once-sacred trees like oak and ash) might awaken these deities and confer their protection. Others note a Christian association with the wood of the cross.
But the origins of this practice are probably much more modern, and banal. In A Dictionary of English Folklore, scholars Jacqueline Simpson and Steve Roud note that the earliest known reference to the practice only dates to 1805. It seems linked to 19th-century children’s games like “Tiggy Touchwood” — types of tag in which children were safe from capture if they touched something wooden, like a door or tree.
In his book The Lore of the Playground, Roud writes: “Given that the game was concerned with ‘protection,’ and was well known to adults as well as children, it is almost certainly the origin of our modern superstitious practice of saying, ‘Touch wood.’ The claim that the latter goes back to when we believed in tree spirits is complete nonsense.”