knock on wood

Example: “We haven't had a single flat tire on this trip, knock on wood.”

Origin: The explanations usually given for this are considerably older than the evidence for the phrase itself. Accounts invoking pagan tree spirits, Celtic reverence for sacred groves, or the wood of the Cross are all repeated confidently, but the expression is only attested in English from around the turn of the twentieth century, with the British form "touch wood" appearing somewhat earlier in the 1800s — far too late to descend directly from ancient practice without a long silent gap nobody can document. A number of folklorists favor a much more mundane source: children's chasing games in which a player was safe from being caught while touching wood, a rule widespread in British schoolyards, which would make the gesture a piece of playground immunity that drifted into adult superstition. American English says "knock," British English says "touch," and the accompanying gesture survives in both regardless of what originally prompted it.