E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The shield of Jupiter made by Vulcan was so called, and symbolised Divine protection. The shield of Minerva was called an gis also. The shield of Jupiter was covered with the skin of the goat Amalthæa, and the Greek for goat is, in the genitive case, aigos. The ægis made by Vulcan was of brass.
I throw my gis over you, I give you my protection.