Read it in some interview with Dio that it was Ronnie's Italian grandmother who first introduced him to the horned up sign: it is to put it pure and simple a ward against the devil. Which means that when ever you do the horned up sign against anyone you are claiming them to be evil and trying to protect yourself...makes you feel like a bit of a twat when especially using it against yourself, especially if you are pagan such as yours truly.
But alas...things, terms, and imperetations change over time: once the word "kaelling" or "kaerring" in Scandinavien meant "little beloved woman" now it means...bitch! And once almost bared female breasts were socially acceptable, while a woman's ancles could make any man blush. So who knows, perhaps horns up has really changed from "uh, oh...I am scared. Jesus, please protect me" to "Hell yeah, rock n roll". As long as no one mentions NWO and Alex Jones, then all is well.
Tortuga's post in Where did the "horns up" hand gesture come from? was marked as the answer