euphemism (n.) Look up euphemism at Dictionary.com
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-) + pheme "speech, voice, utterance, a speaking," from phanai "speak" (see fame (n.)).
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]
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.