E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Crossing the Hand.
Fortune-tellers of the gipsy race always bid their dupe to cross their hand with a bit of silver. This, they say, is for luck. Of course, the sign of the cross warded off witches and all other evil spirits, and, as fortune-telling belongs to the black arts, the palm is signed with a cross to keep off the wiles of the devil. You need fear no evil, though I am a fortune-teller, if by the sign of the cross you exorcise the evil spirit.