The superstition was that disability of any sort was the mark of the devil. The phrases are in languages throughout Europe: the devil's hoof, the devil's horn mark. It reaches back to early Christianity and the middle ages. Where a child was born out of wedlock, the church cooked up the impression that you'd done something sinful, and something dreadful would result. You will still find, particularly in Greece, people doing a little sign when they see a very badly disabled child it needs warding off.