“Knock on wood” came from the belief that spirits resided in trees and knocking would call for their protection or prevent bad luck

“Knock on wood” came from the belief that spirits resided in trees and knocking would call for their protection or prevent bad luck. Over time, the practice evolved into a common superstition to avoid jinxing yourself after saying something. 

Learn more at https://www.history.com/articles/why-do-people-knock-on-wood-for-luck