You’ll have to take this one with a pinch of salt (next idiom anybody?). The phrase finder website did not care to find this phrase, and with Wikipedia currently determined to prove how bad it is to suppress information by suppressing information I am not certain about the reliability of my sources, especially when Google forsook me and I was forced to turn to Yahoo, scarcely an adequate replacement.
Anyway, moving on. Crossing one’s fingers is supposed to bring one luck. It can be traced back to by gone days, when the cross was a symbol of unity and benign spirits were thought to dwell at the intersection point. A wish made on a cross was a way of ‘anchoring’ the wish at the intersection of the cross until the wish was fulfilled.
The superstition, which was once popular among many early European cultures, originally required two people. One well-wisher placed his index finger over the index finger of the person making the wish, with the two fingers forming a cross. Over centuries, the custom was simplified, so that a person could wish on his own, by crossing his index and middle fingers to form an X.
As some legend explains, “Customs once formal, religious, and ritualistic have a way of evolving with time to become informal, secular, and commonplace.” Thus, friends crossing fingers evolved to crossing one’s own fingers, and ultimately to the stock phrase, “Keep your fingers crossed,” with little to no actual finger-crossing at all.
Interestingly, some people cross their fingers behind their back when they are lying. Rather dubiously it was suggested to me by a rather enthusiastic young fellow that the origin of this gesture comes from religious persecution in pre-Christian (but post-Christ… obviously) Rome. In these dark times, when people were asked if they were Christians, they would lie and say no to escape Roman retribution, but make the sign of the cross behind their back to ask God’s forgiveness for the lie. But I thought He was all loving, why then do we need to seek His forgiveness? Alright now, there’s enough of that.
PS: my sister, ‘Nurdy’, wanted me to tell you all that this idiom was her idea.