“Knock wood” or “touch wood” is a superstitious action used to ward off evil consequences or as a charm to bring good luck. It’s most often used when a person makes a statement which seems to tempt fate. The idea is that knocking on wood will ward off evil spirits. You must knock on wood three times after mentioning good fortune or the evil spirits will ruin things for you.
The tradition traces back to an ancient pagan belief that spirits resided in trees, particularly oak, ash, holly or hawthorn, and that by knocking on or touching the wood you were paying a small tribute to them by remembering or acknowledging them and could call on them for protection against ill-fortune. Also, you were thanking them for their continued blessings and good luck.
Greeks worshipped the oak as it was sacred to Zeus, Celts believed in tree spirits, and both believed touching sacred trees would bring good fortune. Irish lore holds that "touching wood" is a way to thank the leprechauns for a bit of luck. Pagans also held similar beliefs of protective tree spirits. Chinese and Koreans thought the spirits of mothers who died in childbirth remained in nearby trees.
A Jewish version traces the origin to the Spanish Inquisition of the 15th century. At the time, persecuted Jews fled to synagogues built of wood, and they devised a coded knock to gain admission. Since this practice spared countless lives, it became common to "knock on wood" for good luck.
This superstition may have been adapted by Christians, as many early pagan beliefs were, and some people associated knocking on wood with the Cross. Even today pieces of wood, or the true cross, are often carried around for good luck.
By the 1800s, many children's games included a refrain to knock on wood. The idea of knocking on wood was probably widespread long before these games were popular. However, the tradition of knocking on wood for luck seemed to become much more widely accepted as these children's games entered the popular imagination. By the 1900s, British and American people were both knocking on wood for good fortune.
It should be noted that in modern times, when many items are manufactured to only appear wood-like, it has become acceptable to knock on a table with a wood veneer, or even on something that is not wood at all, such as plastic or Formica, as long as you say "knock on wood" or "touch wood". To me, this shows that the original reason for knocking is becoming forgotten, even in folk memories, in favor of the knocking action itself.
To my surprise, while doing research for this piece, I came across many references to a person knocking on their own head if no wood was available. This is something I’m in the habit of doing, even when there is wood around. It’s nice to know I’m not the only one.