Ask.Yahoo.com has several explanations for the origin, but writes that they prefer the explanation that spirits dwell in or guarded trees. The Greeks and Celts believed in tree spirits. Irish lore holds that “touching wood” thanks leprechauns for good luck. The Christian explanation is that the cross is the origin of good luck, “…although this is likely a Christian adaptation of earlier pagan practices.” A Jewish version is that persecuted Jews fleeing to synagogues built of wood to escape the Spanish Inquisition used a coded knock to gain admission. The practice saved lives and “…it became common to ‘knock on wood’ for good luck.”