knock on wood

Knocking on wood, and the spoken expression "knock on wood" or "touch wood" are used to express a desire to avoid "tempting fate" after making some boast or speaking of one's own death. The expression is usually used in the hope that a good thing will continue to occur after it has been acknowledged. So, for example, one might say: "The rain looks like it's holding off, touch wood", or "Knock on wood, I'm much better now."

(Wikipedia)

Meaning

The phrase 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 usually used when one is already experiencing some good fortune and hope that it will continue - e.g. "I've been winning on every race - touch wood".

http://www.phrases.org.uk/

• My kids are always impressive weight gainers. Knock on wood.
• "No kidding," Harber said, knocking on the side of his wooden desk.