It's the British equivilent of saying knock on wood.

Contrary to popular belief it has no relation to touching men's wood.

However it is most commonly said in order to prevent a confident statement from bringing bad luck.

Meaning to hope for the best.

When said most people will touch or knock on wood and some strange people will also knock on their heads!
"I haven't been banned from the group yet, touch wood!"
by LottieRawr September 19, 2017
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cockney slang for draw blood
"Do not touch wood from that pig"
by Ampuero December 8, 2006
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