1
general::
Also, keep one's fingers crossed. 1. Wish for luck by crossing two fingers of one hand. For example, I'm crossing my fingers that I get the job, or Keep your fingers crossed that the hurricane goes out to sea. This superstitious statement presumably alludes to the much older practice of making the sign of the cross to ward off evil. [Early 1900s] 2. Tell a white lie that doesn't matter. For example, I told Mom I didn't eat any cookies but I had my fingers crossed. The childish belief that if one keeps one's fingers crossed one may lie with impunity probably comes from children's games in which one was "safe" if one crossed one's fingers, and the ultimate allusion may be the same as in def. 1.
American Heritage Idioms