Knocking on wood is one of those superstitious practices that can quickly become a compulsion, as we constantly find situations arise where we could use a bit of luck.
The origin of this superstition isn't clear- some believe pagans knocked on tree trunks to call upon the spirits that inhabited the trees for protection, others link the wood element to Christ's cross.
But British folklorist Steve Roud believes knocking on wood, or “touch wood” as it's known in the UK, can be traced back to a 19th century kid's game called “Tiggy Touchwood”.
Tiggy Touchwood was similar to tag, but players who touched wood were protected from being tagged, and Steve discusses the connection in his book "The Lore of the Playground":
“Given that the game was concerned with ‘protection,’ and was well known to adults as well as children, it is almost certainly the origin of our modern superstitious practice of saying, ‘Touch wood,’” he argues. “The claim that the latter goes back to when we believed in tree spirits is complete nonsense.”
No matter the origin one thing's for sure- if you start knocking on wood every time you need a bit of luck it can be mighty hard to stop!
-Via History