I was talking to one of my Swedish friends today and the subject of crossing fingers came up. He said that the only thing it means in Sweden is broken promises…the way we used it when we were kids at school. It doesn’t mean to wish for something.
According to various websites that are in no way reputable sources, the tradition of crossing one’s fingers comes from the European, pre-Christian belief that benign spirits lived at the intersection of the two parts of the cross. Therefore wishing on a cross was a way of holding the wish until it came true. Originally, this wishing required two people crossing index fingers to form the cross, but over time the tradition has evolved to the way we know today of wrapping the index finger around the middle finger.* Or, apparently, the middle finger over the index finger, but to me that just looks weird.
As a side note, according to my Russian physicist friend, in Germany (where he lives) instead of crossing their fingers, they press their thumbs.
*I can do this really well on my left hand, but for some reason, the index joint on my right hand is so inflexible, that an attempt to cross the fingers of my right hand ends hilariously with my index finger just lying on top of my middle finger.