What is the origin of the idiom "knock on wood" and when to use it?

The phrase "knock on wood" is a superstitious idiom used to ward off bad luck or to express a hope that something will continue to be lucky or successful. The origin of this phrase can be traced back to ancient pagan and druidic traditions that believed trees had protective spirits. By knocking on wood, people were symbolically calling on these spirits for good fortune or to prevent a jinx.