To cross one's fingers is a hand gesture commonly used to wish for luck. Early Christians used the gesture to implore the protection of the Holy Cross.[1] The gesture is referred to by the common expressions "cross your fingers", "keep your fingers crossed", or just "fingers crossed".
The use of the gesture is often considered by children as an excuse for telling a white lie.[2] By extension, a similar belief is that crossing one's fingers invalidates a promise being made.[3]
^Orange Coast Magazine. Emmis Communications. May 1990. pg. 177. "In early Christian days, a believer confronted by evil or hostile influences implored the power of the Holy Cross for protection by twisting his middle finger over his forefinger and holding the remaining fingers down with his thumb."
^Field Guide to Gestures. Quirk Books. 2003. pg. 201. "Children are a big proponent of this gesture, though they usually use it when telling white lies, believing that having the fingers crossed behind the back makes it okay to fib."