The expression is usually used in the hope that a good thing will continue to occur after it has been acknowledged. So, for example, one might say: “The rain looks like holding off, touch wood”, or “Knock on wood, I’m much better now.”

It is commonly thought that knocking on wood has been a superstitious action to ward off evil throughout history. Some believe it has to do with knocking on the wooden cross. Another explanation for this practice is the pagan belief that spirits (dryads) lived in trees. By knocking on the wood of a tree while making some sort of a bold statement, the speaker could prevent the spirit from hearing him and stop the spirit from interfering.

However, there is no evidence for either theory, and the superstitions have not been traced beyond children’s games of tag of the early nineteenth century.

According to Steve Roud, the earliest documented references to “touching wood” are from 1805 and 1828 and concern chasing games like “Tiggy-touch-wood”, where you are safe from being “tagged” if you “touch wood”. Says Roud, “‘Tiggy-touch-wood’ was an extremely well-known game, and it is most likely that the phrase passed into everyday language.”

[edit] Variations

In some versions of the superstition, it is better to touch the underside of wooden furniture, as this is unfinished and closer to the real wood. Others believe that knocking on the underside of the wooden surface will avoid hitting wood gnomes or tree dryads on their heads, which may anger the creature and counteract the good luck. If there is no wood available, some people will tap their head instead in gentle self-mockery. A more recent development on this motif is the act of younger men knocking gently on their genital region, referencing the slang usage of the word “wood” to mean an erect penis.

In Poland the versions of this charm is “odpukać w niemalowane” [knocking on unpainted (wood)], as the name of the charm suggests the charm only works if one knocks on unpainted wood.

In Switzerland the (German) version is “Holz alange” (touch wood) – but while saying it, knocking on or tapping wood is still required. A simple touch is not enough.

In Sweden, the phrase “ta i trä” (touch wood) is commonly used as a part of the phrase “peppar peppar, ta i trä” (pepper pepper, touch wood), the double “pepper” also being used to ward off a temptation of fate. It’s often shortened to just saying “peppar peppar” while knocking on wood.

In Italy a similar superstition exists, except that one must knock on metal, preferably iron.

In Spain also exists, it’s said “Tocar madera”, and the meaning is the same, a charm to bring good luck.

In Denmark “bank under bordet” (knock under the table) is a commonly used phrase, which is often used as a part of the phrase “7-9-13, bank under bordet”, where “7-9-13” is just another way to say touch wood.

Likewise, in Norway, the term “bank i bordet” (knock the table), is used. In Norway, it is also sometimes used to stress that you’re telling the truth (akin to saying “I swear to god that…”).

Among Christians in Lebanon, knocking on wood is a common expression. Some take it to mean that we thank Christ (of the Cross) for all of our blessings. Often while actually knocking on wood people will say “b-ism as-salib” meaning “By the name of the cross.”

Some tradition has it that knocking up on wood would awaken and release the (benevolent) wood faeries that dwelt there. This is probably based on Germanic forest-dwelling mythology.

“When Saint Helen, mother of Emperor Constantine the Great, visiting the Holy Land and seeking the True Cross upon which Jesus was crucified, found the Precious and Life-Giving Cross, the Cross was venerated by all the faithful in many public processions. Many faithful would come to touch the Holy Cross for blesssing and healing. It was customary to touch the Wood of Life three times (as a confession of faith in the Holy Trinity). This act of touching the True Cross became the earliest recorded histories of ‘knocking on wood.’ Whenever the Holy Cross was put forth for public veneration, touching it, or as English translations render it ‘knocking,’ became common liturgical practice. Once the Holy Cross was transferred to Constantinople and placed in the Great Church of the Holy Wisdom (aka Saint Sophia), Christians continued this piety by touching or knocking on any cross or crucifix (wood was the medium of the day) for blessings and healings. This ancient tradition has been with us for over 1,600 years and has been a pious tradition to this day where people tend to touch anything made of wood … but all interpretations of this behavior point back directly to Jerusalem in the 4th century CE and the True Cross.” (Prof Anastasios Zavales Phd ThD, Ecumenical Patriarchate, Greek Orthodox Archdiocese of the USA).