It was believed gods lived in trees; to ward off bad luck, do this to wood
| Mental Floss [PAR] Apr 16, 2013 ... More recently, simply saying the phrase knock on woodor touch wood ... to 
chase away evil spirits from their homes and trees or to prevent them ... from the 
spirits/gods that lived inside it, or did it after a run of good luck as... [DOC] [TLE] What is the origin of the custom of saying 'knock on wood'/'touch ... [PAR] There is no single theory for this belief.