 If someone says, knock on wood, they're expressing a wish that something will or will not occur. Sometimes phrased as touch wood, it's an expression that dates back to the ancient Druids. They were a race that inhabited England before the Romans, and they worshiped trees, in particular oaks, and held the firm belief that protective spirits lived within trees. They believed trees were sources of good and warded off evil spirits. People in need of good luck would go and touch a tree.