The phrase "touch wood", "knock on wood" or equivalent has been purportedly used in Italian, Old English, Romanian, Bulgarian, Polish and Turkish folklore, among others. Generally, explanations of its origins are sketchy or unsubstantiated.
Do we have any early records of where and how it originated, and the context in which it was originally used?
I have seen several attempts to unravel this, but most explanations are speculative: the wood being a way to thank the elves for good fortune (and a plea to continue it), a reference to the Cross of the Crucifixion, a pre-conversion expression recognizing spirits inside trees.
This site indicates that the oldest recorded references are from the turn of twentieth century. It correctly points out that pre-industrial people would have looked for a piece of iron to ward off dangerous spirits and the outcomes they might bring. I have seen folklorists speculate that touching wood was part of a nineteenth-century children's game, but even that is speculative. As with so many folk practices, finding origins can be elusive.
In Turkey we pull our ear lobe make and knock on wood 2 times to ward off jinx/bad luck (well i mean we as turkish ppl, im not superstitious)