Crossed fingers
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". 
The use of the gesture is often considered by children as an excuse for telling a white lie. By extension, a similar belief is that crossing one's fingers invalidates a promise being made.

Origin
Historically, the origins of this sign may be unclear, although it was likely propagated through the supposition of its likeness to the Cross of Christ.
In some sources it is claimed that in 16th-century England, fingers would be crossed in place of the Sign of the Cross to ward off evils and ill health.The act of crossing fingers for luck or to ward off ill luck is a well-understood gesture in the UK and parts of Scandinavia, but less so in mainland Europe. Its earliest recorded reference dates back to 1912 and is linked to the ladder superstition. The gesture is still used in school playgrounds as a protective action. The popular explanation for its origin, dating back to classical times when Christians were persecuted, is unfounded due to its late appearance, restricted distribution, and lack of relation to the shape of a cross.

Related gestures
In Vietnam the gesture is considered rude, especially to another person. 
Referring to female genitals, it is comparable to the middle finger in American culture.
In German-speaking countries and also Sweden and Latvia the gesture is a sign of lying. Instead, wishing for luck is gestured by holding thumbs. The same gesture is used in many Slavic countries such as Poland, the Czech Republic, Slovakia, Bulgaria and ex-Yugoslav republics. In South Africa, Afrikaans speakers also have the related phrase "duim vashou" meaning "holding thumbs tightly".
In pre-Christian Western Europe, a related gesture had two people crossing their index fingers to form a cross, which represented perfect unity; this gesture was used to make wishes.

See also
Benediction
God bless you
Orans
Sacramentals
Sfiga
Holding thumbs

References
External links
 Media related to Fingers crossed at Wikimedia Commons