http://www.worldwidewords.org/qa/qa-tou1…

“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.”

Source(s):

http://www.worldwidewords.org/qa/qa-tou1…

ck

But beware of the many sources that claim to have traced it back to a specific religion or cult. They’re just shaky guesses that make the average paperback history of Wicca look like a model of scholarship by comparison.

The general practice of knocking on or touching wood is widespread across Europe, where it presumably originated. It also appears to be quite old, reportedly having been pondered by medieval scholars, though I was unable to find any primary source documenting that.

The generic guess is that the original idea was to go knock on a tree to get the attention of a nature spirit thought to live inside. Various trees and the wood thereof were regarded as mystical in European paganism, so the idea is reasonable. (It also doesn’t have to be an old practice, since this kind of animist thinking has always been around.)

Another common guess is that it was an early Christian practice related to touching a crucifix or wooden cross. Without knowing the actual age of the practice, we can’t really judge this claim. But I’m suspicious because of Christianity’s long practice of recasting paganisms in Christian terms. And who would knock on a cross?

Some Jews, incidentally, avoid using “knock on wood” because of this supposed Christian origin. There’s even an alternative Jewish origin—a supposed secret door-knocking code used by Jews escaping the Inquisition. But this literalist explanation seems like a thin attempt to make the practice palatable to Jews.

Yet another suggestion is that it relates to a game of tag that involved touching a tree to be “safe”—though claims here range from recent children’s games to ancient Native American contests.

In the 1800s, tag was commonly known as “touch,” and versions using a wooden or metal object as home base were respectively called “touch wood” and “touch iron.”

Interestingly, the British version of “knock on wood” is “touch wood.” It’s unclear if the game name had anything to do with this. (“Touchwood,” a 1600s term for rotten wood used as fuel, seems an even less likely candidate for inspiration.)

Germans and Norwegians have “knock on wood” sayings just like Americans, so it’s curious that the British have the more passive “touch.” It’s possible the “touch wood” game name somehow replaced “knock on wood” in Britain, but no one really knows.

In truth, the origins (and there may be many independent origins involved) are probably lost forever, and the generic practice of touching or banging on wood has probably gone through many formulas and phrases over the centuries.

It’s clear that today’s superstitious wood-knockers (or more likely, Formica-knockers) think they’re doing it for a variety of reasons—Christian, pagan, Jewish or just habitual. It’s impossible to say any of these are “wrong” reasons; like all superstition, the real origin is inside the person’s mind.

http://answers.yahoo.com/question/index?qid=20070502195211AAkkEKR&fr=hp