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]
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.