Etymology of "Holding Thumbs"

[SLA Webmaster]

A query from a visitor to our website:
"Could you please help direct me to the etymology for the South
African idiom "I'm holding my thumbs for you"? It means good luck.
It's my understanding that crossing one's fingers harkens back to
evoking Christ's cross, as does touching wood. What is the cultural
root for holdling one's thumbs? Thank you!" -- Dave

Can anyone help Dave?

(A similar expression exists in French: «je te tiens les pouces» means
something akin to "I'm hoping you'll get what you want." -- Alex)

-- 
Alex Enkerli
SLA Web Guru