To knock on something made of wood to keep from having bad luck

I bought a used car and so far I've had no expensive repairs to do, touch wood!

Ho comperato una macchina usata e finora, toccando ferro, non ho avuto riparazioni costose da fare

They announced on TV that the flu virus will be a very dangerous strain this year. Let's knock on wood that there won't be deaths among children and old people

Dicono che l'epidemia di influenza quest'anno sarà molto lunga e pericolosa. Tocchiamo ferro e speriamo che non colpisca bambini e anziani!

Of the many explanations about the origin of this expression, the one of knocking on a tree trunk, believed to contain friendly guardian spirits, seems to be the most popular. A second theory is that the practice refers to a chasing game, like in the children's game of tag: those who touch wood are safe from capture. A third suggestion is that the wood may refer to the wooden crucifix of Jesus Christ: in the Middle Ages there were in circulation pieces of the Holy Rood or Cross on which Jesus was crucified, and to touch it would be to seek divine protection. An old British proverb tells us: "Touch wood; it's sure to come good"