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

Although the origins of knocking on wood are thought to date back to the druids and their worship of trees, there is no evidence of this (obviously). The earliest documented references of knocking on wood date back to 1805 and relate to certain children's chasing games such as 'tiggy touch wood' where you were safe from being 'tagged' if you touched wood.

one theory is that in pagan people belived that spirits lived in the trees.by knocking on wood while saying something you stopped the spirit in the tree from hearing you and stop him from interfering.another comes from an old game simular to tag,called tiggy-touch wood.in the game you were sdafe from being tagged if you were touching wood.

=O I'll never beat that but, My last name is Wood, so.... Knock on my hollow head?