knock on wood

[knock on wood] {v. phr.} To knock on something made of wood tokeep from having bad luck. - Many people believe that you will havebad luck if you talk about good luck or brag about something, unlessyou knock on wood; often used in a joking way.

Charles said, "Ihaven't been sick all winter." Grandfather said, "You'd better knockon wood when you say that."