To cross one's fingers, is an apotropaic  hand gesture, used to superstitiously

Superstition

Superstition is a belief in supernatural causality: that one event leads to the cause of another without any process in the physical world linking the two events....

 wish for good luck or to nullify a promise.

The gesture is referred to by the common expression "fingers crossed", meaning "let's hope for a good outcome".

The 1787 Provincial Glossary of popular superstitions by Francis Grose

Francis Grose

Francis Grose was an English antiquary, draughtsman, and lexicographer. He was born at his father's house in Broad Street, St-Peter-le-Poer, London, son of a Swiss immigrant and jeweller, Francis Jacob Grose , and his wife, Anne , daughter of Thomas Bennett of Greenford in Middlesex...

records the recommendation to keep one's fingers crossed until one sees a dog to avert the bad luck attracted by walking under a ladder.