A hand gesture, sometimes performed by two people together, commonly used to wish for luck, but also used by both children and adults when telling a white lie, in the belief that doing so means they won’t get caught.
Wikipedia says that the gesture of crossed fingers traces back to the early Christian Church. Members of that church, when faced with evil, would cross their fingers in order to invoke the power associated with the Christian cross for protection. Also, when persecuted by the Romans, Christians used the symbol of crossed fingers in order to recognize one another and gather for worship services. This idea is further supported by noting that the gesture is not common among Muslim or Buddhist cultures.
In 16th century England, people crossed fingers or made the sign of the cross in order to ward off evil, as well as colds, since they also did so when people coughed or sneezed. And I suspect some people still do that, though I believe that a hot rum toddy would be far more effective.
The superstition was popular among many early European Christian cultures. In some places, a comrade placed his index finger over the index finger of the person making the wish, the two fingers forming a cross. One person makes the wish, the other empathizes and supports. Over centuries, the custom was simplified, so that a person could make the wish on his own, by crossing his index and middle fingers to form an X. But two people hooking index fingers as a sign of greeting or agreement is still common in some circles today.
Some people suggest that the act of crossing one’s fingers as a sign of luck or making a wish traces back to pre-Christian times, speculating that the cross was a symbol of unity and that benign spirits dwelt 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.
Another source suggests that the modern version of crossing your fingers came into existence during the Hundred Years War. In this epic conflict between France and England, which lasted from 1337 to 1457, the rival armies needed all the luck they could get. The archer preparing to make a shot would have crossed his fingers and then said a prayer before pulling the bowstring.
That may be so, but I doubt that any archer, in the heat of battle, would stop long enough to cross fingers and pray. The opposing soldiers would hardly be so polite as to let him perform such a ritual.