From Longman Dictionary of Contemporary Englishknock on woodknock on woodAmerican English used to say that you hope your good luck so far will not change SYN touch wood British English → knock
Examples from the Corpus
knock on wood• I haven't had a cold all winter, knock on wood.• Hopefully, knock on wood, Lieby is going to be in there most of the time.• Always ready to knock on wood, throw salt over my shoulder, bite my tongue, cross my fingers.• KineHUHre is used as a verbal device similar to knocking on wood to ward off evil forces. 