What is the shield of Zeus?
According to Greek mythology, Aegis was the name of the shield carried by Zeus, as well as (more often) his daughter Athena. Some legends represent it as a garment of goatskin, or an armored breastplate. The gist of the legend, though, is that the Aegis is a protective device. Consequently, it has come to mean in standard English the protection or support of another.
—The Fact Monster