The phrase originates based on a superstition that knocking or touching wood will ward off evil spirits. Wood and trees have an association with good spirits in mythology. It was considered good luck to tap trees to let the good spirits know that you were there. The British version of the phrase is « touch wood », while the American version is « knock on wood ». The phrase originated in latin in the early 17th century, and came into English (British version touch wood) by 1850. The American version knock on wood was known from the early 20th century.