What Does it Mean to 'Knock On Wood'?

'Knock on wood' is an idiom meaning to say for something not to happen, or maybe after it happened.

For example:

"We will win this basketball game, knock on wood."

In this case, he means that he doesn't want the opposite to happen; losing.

There was a superstition that if you say you wish that something would happen, evil spirits will make sure that exactly the opposite will happen. Knocking on wood supposedly prevented this. Instead on actual knocking, people started saying "knock on wood", it became an idiom.