knock on wood
Definition from Wiktionary, the free dictionary
- (RP): IPA: /ˈnɒk ɒn ˈwʊd/
- (US): IPA: /ˈnɑk ɑn ˈwʊd/, /ˈnɔk ɔn ˈwʊd/, /ˈnak ɑn ˈwʊd/
Audio (US) (file)
- (idiomatic, US) To take a customary action to ward off some misfortune that is believed to be attracted by a presumptuous statement.
- (idiomatic, US) A self-directive to undertake the customary action to ward off bad luck.
- I do a lot of walking, and I have never had any knee problems yet, knock on wood.
See also 
- touch wood (UK)
take a customary action to ward off misfortune