An expression said when knocking on wood in order to keep from having bad luck.
I am sure that your tax returns will not be audited, knock on wood.
One theory is that it originated in the middle ages when there were in circulation, pieces of the Holy Rood or Cross on which Jesus was crucified. To touch one of these was supposed to bring good luck hence touch wood for good luck.
Alternative: The Druids, who worshipped trees, especially Oaks, wore a piece of Oak around their neck to ward off evil spirits. Hence touch wood for good luck.
Alternative: You knock on wood because of those wily Wood Sprites. In medieval times, people believed in mischievous creatures known as sprites. Sprites are actually spirits or ghosts who were reputed to enjoy causing trouble and wreaking havoc in the lives of the living. Among the most mischievous were wood sprites.
If you were to mention something good, the wood sprites would try to foul it up. The thought was that if you knocked on the wood when you said these things, the wood sprites would not be able to hear you because of the knocking sound. Hence they would leave you alone.