Isn't is also an Italian sign?
Perhaps the most polite version I've heard is that it's an indicator of marital unfaithfulness. But I suspect that translation was sweetened up for me a tad
I think it is also a mudra. It is said that Dr Usui used to carry out his reiki treatments with his hand in this particular mudra - the understanding being that the two fingers acted as an antenna to channel the reiki energy more effectively