Mano Cornuto means “horned hand” in Italian; the gesture is commonly depicted on charms against the evil eye. It is unclear whether the gesture originated as an image of horns or as a “poking out the eyes” gesture (against the malocchio or “evil eye”), but ancient lunar goddess charms depicting animal horns were used for similar protective purposes and are probably related to the gesture.
The use of the horns as a symbol of satanic belief is recent, and is evolved from its use by heavy metal musicians and fans. (A good discussion of the use of the horned hand gesture in rock can be found here.) The horned Hand gesture is also used occasionally by Wiccans as a symbol of the “horned God” or as the horns of the Moon Goddess, depending on tradition.