Knock on wood
Meaning

The phrase voiced by people who rap their knuckles on a piece of wood hoping for good luck. In the UK the phrase 'touch wood' is used.

Origin

May be the association that wood and trees have with good spirits in mythology, or with the Christian cross.
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Meanings and Origins of Phrases, Sayings, Idioms and Quotes
http://www.phrases.org.uk/meanings/index.html