The Answer:

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