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Meaning: This phrase is used by people who rap their knuckles on a piece of wood hoping to stave off bad luck. In the UK, the phrase ‘touch wood’ is used – often jokingly by tapping one’s head. The phrases are sometimes spoken when a person is already experiencing some good fortune and hope that it will continue – for example “I’ve been winning on every race – touch wood”
Read more with: http://www.phrases.org.uk/meanings/knock-on-wood.html