To knock on something made of wood to keep from having bad luck. — Many people believe that you will have bad luck if you talk about good luck or brag about something, unless you knock on wood; often used in a joking way.

敲木头做的东西以防止有坏运气。 - 许多人认为如果谈论好运气或夸口时会走霉运，除非你敲下木头; 经常是开玩笑的方式。

Next year, I will be promoted to president of my company – knock on wood. 明年，我将晋升为公司总裁 - 敲木头。