Summed up:

1. From tree worshipping cultures

2. The cross

3.Jewish historical event – people knodking on wooden door in code….saved many lives…became good luck symbol.

In conclusion..no one was really sure which was the main reason.

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.