The informant is a 27-year-old that grew up in Madison Wisconsin and currently lives in Southern California. He has followed this superstition for as long as he can remember and believes he learned it from his older brother when he was young. He takes it very seriously and anytime he catches himself boasting about something prematurely, he will make sure to knock on wood.
The Superstition involves a person knocking on wood in order to prevent something that had recently been said from not coming true. The practice is typically seen when someone says something boastful or implies that something good will happen to them with certainty. They will then knock on a piece of hardwood nearby as a way to ward off any jinxing that may have occurred by them stating this out loud.
This folk belief is one of superstition. While superstition tends to have a negative connotation and be viewed as a pejorative term among many people, I think superstitions are usually a positive thing that people partake in. This ‘knock-on wood’ superstition that the informant discussed with me is particularly positive. The practice of doing something to sort of acknowledge an overzealous statement that one has made is a good thing for people to partake in. While it may or may not have any impact on the outcome of a certain event, it forces people to address hubris and maintain a humbler balance of how they speak about things. The literal ‘knock-on wood’ superstition reminds me of another common trope that I have heard many times throughout my life that goes “Don’t count your chickens before they hatch”. This tope relates to overzealous farmers that assumed that were going to have as many chickens as they had eggs.
For another version, see Evan Andrews, Aug 22. 2018, Why do people knock on wood for luck?