The superstitious phrase ‘touch wood’ is one with many contested origins and it is still not clear which is the correct one. In the UK, many people still touch wood and use the phrase when they talk about their luck or mention their own death and want to stop their words from “tempting fate” and jinxing future good fortune. In the US the phrase ‘knock on wood’ is more commonly used.
Many believe that the phrase derives from the pagan belief that good and evil spirits lived in trees and touching the wood of a tree would gain the blessing of the spirits and stop them bringing misfortune on the speaker. The idea of the two ‘knocks’ is that one knock on the tree would ask the spirits for good luck and the second knock would thank the spirits.
Another interpretation of the phrase is that it is Christian, possibly adapted from the pagan original. The wood could refer to the Cross and thus touching wood meant that you were requesting the blessing of God.
However old the original custom is, it appears that the established phrase is relatively modern. The first recorded use of ‘touch wood’ is in 1899 and ‘knock on wood’ in 1905.