Where did the term "knock on wood" come from?

To touch wood or knock on wood is a superstition action to ward off any evil consequences or bad luck, perhaps because of some recent action you’ve taken or untimely boasting about your good fortune (“I’ve never been in danger of drowning, touch wood”); it can also be a charm to bring good luck.

The origin is unknown, though some writers have pointed to pre-Christian rituals involving the spirits of sacred trees such as the oak, ash, holly or hawthorn. There is, I’m told, an old Irish belief that you should knock on wood to let the little people know that you are thanking them for a bit of good luck. There’s also a belief that the knocking sound prevents the Devil from hearing your unwise comments. Others have sought a meaning in which the wood symbolises the timber of the cross, but this may be a Christianisation of an older ritual. It wasn’t always wood that was lucky: in older days, iron was also thought to have magical properties, and to touch iron was an equivalent preventative against ill-fortune.

The phrase itself is relatively modern, as the oldest citation for the British version of the phrase, touch wood, that I can find dates only from 1899. The American equivalent knock on wood is roughly contemporary, with my first example from 1905.
The phrase "knock on wood" is believed by many to have come from the Catholic Church, when old-time rosaries and crucifixes were made from wood and were used frequently during times of stress and trouble.
Therefore, holding onto or rubbing the wooden rosary became a common way for Christians to deal with hardships and the term eventually evolved into "knock on wood" for good luck.
The term is actually "touch wood" the origin is believed to date from the time when man regarded trees as powerful "wood spirits" and thought by touching them reverently you gained their protection, as time passed the belief has been broadened and wood in any form is included . The act is also associated with mans dread of boasting or of appearing too sure about something for fear of offending the gods so touching wood is to pacify the gods.As well as wood, iron can serve the same purpose though touching your own head will not achieve anything,but maybe a laugh. Also in some circles because of Christs crucifixion on a Cross of timber this gave all wood an aura of holiness.
"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."
We say "Touch wood" - nearer to the original pagan appeal for granting wishes - "Touch a tree".

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