What is the origin of the idiom 'to hold thumbs'?

My understanding is that it's a British version of America's "Cross your fingers," which basically means, "Hope that this happens."

I also assume that the origin comes from the physical act of taking your thumb and placing it in the palm of your hand with the tip of the thumb pointing out from between your middle finger and your index finger (as your fingers are wrapped down in a fist gesture). Since usually this is done in both hands while giving the gesture of "Holding thumbs," the plural is used since you're doing it with both thumbs.

This expression may also be used in the Netherlands, leading to the South African use of it as discussed above. The expression to "Cross your fingers" is common in the UK and parts of Europe and has been used there for centuries.