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-.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit gaus, Greek bous, Latin bos, Old Irish bo, Latvian guovs, Armenian gaus, Old English cu, German Kuh, Old Norse kyr, Slovak hovado "cow, ox."
In Germanic and Celtic, of females only; in most other languages, of either gender. For "cow" Latin uses bos femina or vacca, a separate word of unknown origin. Other "cow" words sometimes are from roots meaning "horn, horned," such as Lithuanian karvė, Old Church Slavonic krava.
<a href="https://www.etymonline.com/word/euboea">Etymology of euboea by etymonline</a>
Harper, D. (n.d.). Etymology of euboea. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/euboea