What's the horns of the devil mean?

If you are talking about the Baphomet sigil it means duality.If you are talking about common orthodox religion it is because Devils are depicted with horns in a attempt to demonize the old religions of fertility that were spread throughout the west european countries.Greece is a prime example, christian-judaic concepts made early contact with Greece as they traveled west, and a well known god at the time was Pan, it is his image that supplies us with one of the classic images of Satan.Astrology signs with horns on them simply represent fertility, or masculine symbols.