You got it wrong. I think the gesture means "knock on wood". Following is what I googled:



[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 superstition 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.

Thanks a lot!

knock on wood = cross my finger