The gesture of putting your index and middle fingers across each other as a sign of hoping for good luck is a scaled-down version of the Christian one of making the sign of the Cross with your whole hand and arm as a request for divine protection. It is also superstitiously employed when telling a deliberate lie with the idea of warding off the evil that might be expected to befall a liar.
1998 - Spectator - Since resources were limited the only hope the clients had was to hang in there, fingers crossed.
RELATED IDIOMS :
at cross purposes
misunderstanding or having different aims from one another
cross as two sticks
very annoyed or grumpy - British informal
This expression is a play on the two senses of cross, firstly 'bad-tempered' and secondly intersecting.
cross the floor
join the opposing side in Parliament – British
The floor of the House of Commons is the open space separating members of the Government and Opposition parties who sit on benches facing each other across it.
cross my heart
used to emphasize the truthfulness and sincerity of what you are saying or promising – informal
The full version of this expression is cross my heart and hope to die and is sometimes reinforced by making a sign of the Cross over your chest.
cross his palm with silver
pay someone for a favour or service – often humorous
Crossing someone's palm with silver was originally connected with the telling of fortunes when the client would literally trace out the sign of a cross on the hand of the fortune-teller with a silver coin.
cross the Rubicon
take an irrevocable step
The Rubicon was a small river in north-east Italy which in the first century BC marked the boundary of Italy proper with the province of Cisalpine Gaul. By taking his army across the Rubicon into Italy in 49 BC, Julius Caesar broke the law forbidding a general to lead an army out of his own province and so committed himself to war against the Senate and Pompey.
have an argument or dispute
Originally, this expression had the literal sense of fight a duel.
have your cross to bear
suffer the troubles that life brings
The reference here is to Jesus (or Simon of Cyrene) carrying the Cross to Calvary before the Crucifixion. The image is also used metaphorically in the New Testament (for example, in Matthew 10 : 38 : And he that taketh not his cross and followeth after me is not worthy of me).
get your wires crossed = get your lines crossed
have a misunderstanding.
Wires being crossed originally referred to a faulty telephone connection (a crossed line) which resulted in another call or calls being heard.
be caught in the crossfire
suffer damage or harm inadvertently as the result of the conflict between two other people or groups.
The literal sense of the phrase, in a military context, is be trapped (and possibly killed) by being between two opposing sides who are shooting at each other.
1998 - New Scientist - This suggested that the corneal cells are innocent victims caught in the crossfire as T cells fight the viral infection.
at a crossroads = at the crossroads
at a critical point when decisions with far-reaching consequences must be made
dirty work at the crossroads
illicit or underhand dealing – humorous
This expression is recorded from the early 20th century and may reflect the fact that crossroads, the traditional burial site for people who had committed suicide, were once viewed as sinister places.
1914 - P. G. Wodehouse - The Man Upstairs – A conviction began to steal over him that some game was afoot which he did not understand that - in a word - there was dirty work at the crossroads.