"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]
digraph in certain Greek or Latin words; it developed in later Latin where classical Latin used separate letters. The Latin digraph also was used to transliterate Greek -ai- (as in aegis). When Latinate words flooded English in the 16c. it came with them, but as an etymological device only, and it was pronounced simply "e" and eventually reduced to that letter in writing (as in eon, Egypt) in most cases, excepting (until recently) proper names (Cæsar, Æneas, Æsculapius, Æsop). When divided and representing two syllables (aerate, aerial) it sometimes is written aë.
"female monster with a petrifying look," late 14c., in Greek legend, any of the three hideous sisters, with writhing serpents for hair, whose look turned beholders to stone, from Greek Gorgones (plural; singular Gorgō) "the grim ones," from gorgos, of a look or gaze, "grim, fierce, terrible," later also "vigorous, lively," a word of unknown origin. Beekes' sources reject the proposed connections to Old Irish garg "raw, wild," Old Church Slavonic groza "shiver," Armenian karcr "hard."
Transferred sense of "terrifyingly ugly person" is from 1520s. Their names were Medusa, Euryale, and Stheino, but when only one is mentioned, Medusa usually is meant. Slain by Perseus, her head was fixed on the aegis of Athena.