word-forming element, in modern use meaning "good, well," from Greek eus "good," eu "well" (adv.), also "luckily, happily" (opposed to kakos), as a noun, "the right, the good cause," from PIE *(e)su- "good" (source also of Sanskrit su- "good," Avestan hu- "good"), originally a suffixed form of root *es- "to be." In compounds the Greek word had more a sense of "greatness, abundance, prosperity," and was opposed to dys-.
Greek goddess of victory (identified by the Romans with their Victoria), literally "victory, upper hand" (in battle, in contests, in court), probably connected with neikos "quarrel, strife," neikein "to quarrel with," a word of uncertain etymology and perhaps a pre-Greek word. As the name of a type of U.S. defensive surface-to-air missiles, attested from 1952. The brand of athletic shoes and apparel, based near Portland, Oregon, has been so known since 1971, named for the Greek goddess, having been founded in 1964 as Blue Ribbon Sports.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit asmi, Hittite eimi, Greek esti-, Latin est, Old Church Slavonic jesmi, Lithuanian esmi, Gothic imi, Old English eom, German ist.