Wiktionary
adv. 1 (alternative spelling of fingers crossed English) (hopefully, wishfully) 2 (en-pastcross one's fingers) (hoped, wished) n. 1 a superstitious hand gesture of good luck 2 to express that two people are very close
Wikipedia
To cross one's fingers is a hand gesture commonly used to wish for luck. Occasionally it is interpreted as an attempt to implore God for protection. The gesture is referred to by the common expressions "cross your fingers", "keep your fingers crossed", or just "fingers crossed". Some people, mostly children, also use the gesture to excuse their telling of a white lie. By extension, a similar belief is that crossing one's fingers invalidates a promise being made.
Usage examples of "crossed fingers".
He winked conspiratorially and held up his own crossed fingers as we filed up on to the stage and prepared to confront the bored ranks of our raw material.
Duffy glanced toward the prostrate Fisher King, who smiled wanly and held up two crossed fingers.
Do you tell them to fight fair, no tricks or crossed fingers behind their back?
He raised his hands toward his face and placed his crossed fingers against his skull, where he had inserted the bit of tooth on Marta and Aarundel.
Hermione stood up, her crossed fingers in her mouth, as Harry streaked toward the ground like a bullet.