see Euxine.

1650s, from Greek euphemismos "use of a favorable word in place of an inauspicious one, superstitious avoidance of words of ill-omen during religious ceremonies," also of substitutions such as Eumenides for the Furies. This is from euphemizein "speak with fair words, use words of good omen," from eu- "good, well" (see eu-) + phēmē "speech, voice, utterance, a speaking," from phanai "speak" (from PIE root *bha- (2) "to speak, tell, say"). See also Euxine, and compare Greek Greek aristeros "the better one," a euphemism for "the left (hand)." In English, a rhetorical term at first; broader sense of "choosing a less distasteful word or phrase than the one meant" is first attested 1793. Related: Euphemistic; euphemistically.

All the ancients, but most of all the Athenians, were careful not to use ill-omened words; so they called the prison 'the chamber,' and the executioner 'the public man,' and the Furies (Erinyes) they called 'Eumenides' ('the kindly ones') or 'the Venerable Goddesses.' " [Helladius of Antinoopolis, 4 c. C.E., quoted by Photius]

Thus, in our dialect, a vicious man is a man of pleasure, a sharper is one that plays the whole game, a lady is said to have an affair, a gentleman to be a gallant, a rogue in business to be one that knows the world. By this means, we have no such things as sots, debauchees, whores, rogues, or the like, in the beau monde, who may enjoy their vices without incurring disagreeable appellations. [George Berkeley, "Alciphron or the Minute Philosopher," 1732]

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-.

Proto-Indo-European root meaning "stranger, guest, host," properly "someone with whom one has reciprocal duties of hospitality," representing "a mutual exchange relationship highly important to ancient Indo-European society" [Watkins]. But as strangers are potential enemies as well as guests, the word has a forked path.

The word ghos-ti- was thus the central expression of the guest-host relationship, a mutual exchange relationship highly important to ancient Indo-European society. A guest-friendship was a bond of trust between two people that was accompanied by ritualized gift-giving and created an obligation of mutual hospitality and friendship that, once established, could continue in perpetuity and be renewed years later by the same parties or their descendants. [Calvert Watkins, "American Heritage Dictionary of Indo-European Roots"]

It forms all or part of: Euxine; guest; hospice; hospitable; hospital; hospitality; hospodar; host (n.1) "person who receives guests;" host (n.2) "multitude;" hostage; hostel; hostile; hostility; hostler; hotel; Xenia; xeno-; xenon.

It is the hypothetical source of/evidence for its existence is provided by: Greek xenos "guest, host, stranger;" Latin hostis, in earlier use "a stranger," in classical use "an enemy," hospes "host;" Old Church Slavonic gosti "guest, friend," gospodi "lord, master;" Old English gæst, "chance comer, a stranger."