Crossing the fingers started out as a way of making the sign of the Christian cross with the fingers of one hand. The gesture was used to wish for luck which is where the “I hope this will work!” usage comes from.
Sometimes it can mean asking the Christian god for protection. People who told minor lies could view mysterious spiritual forces to retaliate for their lying, so crossing the fingers was viewed as a way to protect themselves from such retribution. By extension it was then viewed to invalidate the promise.
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