"protection," 1793, a figurative use of Latin aegis, from Greek Aigis, the name of the shield of Zeus, a word said by Herodotus to be related to aix (genitive aigos) "goat," from PIE *aig- "goat" (source also of Sanskrit ajah, Lithuanian ožys "he-goat"), as the shield was of goatskin. Athene's aigis was a short goat-skin cloak, set with a gorgon's head and fringed with snakes. The exact use and purpose of it is not now clear.
The goatskin would be worn with the two forelegs tied in front of the wearer's breast, or possibly with the head passed through an opening made at the neck, by the removal of the animal's head. [F. Warre Cornish, ed., "Dictionary of Greek and Roman Antiquities," London, 1898]
updated on January 30, 2018