It was believed gods lived in trees; to ward off bad luck, do this to wood
[DOC] [TLE] Knocking on wood - Wikipedia [PAR] Knocking on wood - in Britain the phrase is "touch wood" - is an apotropaic 
tradition of literally touching, tapping, or knocking on wood, or merely stating that 
one is doing or intend to do so, in order to avoid ... Iranians traditionally believe 
knocking on the wood wards off evil spirits.