The gesture has a long and complex history, undoubtedly originating as a manual representation of the Devil's horns; Bram Stoker referred to it in his novel Dracula:
When we started, the crowd round the inn door, which had by this time swelled to a considerable size, all made the sign of the cross and pointed two fingers towards me. With some difficulty I got a fellow-passenger to tell me what they meant; he would not answer at first, but on learning that I was English, he explained that it was a charm or guard against the evil eye.
It has subsequently been co-opted by musicians, athletes, politicians, and celebrities for a variety of purposes and meanings. Students at several universities use the sign in support of their team. In baseball and football it can mean "two outs" or "second down." It is even reportedly an unofficial sign for "B.S." (as the horns of a bull) in American sign language!
Photo via Old Hollywood.