Long ago, when people were very superstitious, the sign of the cross had almost a magical meaning for good luck. If two people wished for the same thing to happen, one held out his index finger and the other placed his index finger across it, making the sign of a cross.
As time passed, this belief continued, but the procedure changed so that one person alone could wish for his own good luck without needing another person to wish for it too. All that person had to .do was cross his middle finger with his index finger and make his wish.
Today we say, “I’ll keep my fingers crossed,” and it means we hope that a wish will come true, or that nothing bad will happen to spoil our good luck.