"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.