a mountain nymph," "1580s, from Latin Oreas (genitive Oreadis), from Greek Oreias "mountain nymph," from oros "mountain, height," also (in Egypt) "desert," in contrast to the cultivated plain, probably from PIE *heros- "elevation." Also see -ad. Attested in English from late 14c. in Latin plural form Oreades. An Old English word for a similar thing was dunælfa ("mountain elf").
<a href="https://www.etymonline.com/word/Orestes">Etymology of Orestes by etymonline</a>
Harper, D. (n.d.). Etymology of Orestes. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/Orestes