Not relevant just to Asian faiths, as it was a symbol widely used in many Indo-European cultures, particularly Slavic and Roman(and Germanic, but that perhaps hits too close to the problem). You still can see it as ornament carved into wooden beams under a roof and above doors on many old houses in rural areas of Serbia and Romania. Not sure though if it had any real superstitious meaning after the Christianity came, or it was just a traditional ornament that continued to be used purely as a decoration.