Wednesday's Random Slang-o-rama: Shaking the cross

Here's a fun one for the books (truly: this bit of slang actually appears in Mark Twain's Life on the Mississippi).

Can you venture a guess as to what Mr. Twain meant when he talked about "shaking the cross?"
Hint: It has nothing to do with warding off vampires.
I'll give you a moment to ponder.

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Time's up and coffee's getting cold!

Hang onto your wallets, because here is the definition, according to Americanisms, Old and New:

TO SHAKE THE CROSS—Thieves' argot. To quit stealing.

And here is the passage from Life on the Mississippi:

"The day my time was up, you told me if I would shake the cross and live on the square for three months, it would be the best job I ever done in my life."

A couple of fellows who need to think seriously about "shaking the cross" before they get caught. ["Dandy PickPockets Diving: Scene Near St. James Palace" Published in The caricature magazine, or Hudibrastic mirror, by G.M. Woodward, vol. 5, Foilio 75. Date: December 2, 1818]