Touch wood/knock on wood
Q From Mike Gast: What is the origin and true meaning of knock on wood or touch wood?
A To touch wood or knock on wood is a superstitious action to ward off any evil consequences or bad luck, perhaps because of some recent action you’ve taken or untimely boasting about your good fortune (“I’ve never been in danger of drowning, touch wood”); it can also be a charm to bring good luck.
The origin is unknown, though some writers have pointed to pre-Christian rituals involving the spirits of sacred trees such as the oak, ash, holly or hawthorn. There is, I’m told, an old Irish belief that you should knock on wood to let the little people know that you are thanking them for a bit of good luck. There’s also a belief that the knocking sound prevents the Devil from hearing your unwise comments. Others have sought a meaning in which the wood symbolises the timber of the cross, but this may be a Christianisation of an older ritual. It wasn’t always wood that was lucky: in older days, iron was also thought to have magical properties, and to touch iron was an equivalent preventative against ill-fortune.
The phrase itself is relatively modern, as the oldest citation for the British version of the phrase, touch wood, that I can find dates only from 1899. The American equivalent knock on wood is roughly contemporary, with my first example from 1905.