The ancient Greeks had a euphemism for euphemism itself. Sometimes the best-omened word was no word at all.

The English term comes from Greek euphemismos, the use of a favorable word in place of an inauspicious one. Etymonline traces it to the verb euphemizein, "speak with fair words, use words of good omen." The parts are eu-, "good, well," and pheme, "speech, voice, utterance." Wikipedia adds that pheme could also mean prophetic speech or rumor.

The setting was religious. Etymonline quotes a 4th-century writer who says the ancients, especially the Athenians, avoided ill-omened words. Wikipedia notes that the word itself was used to mean "to keep a holy silence," speaking well by not speaking at all.

The habit shaped real names. The Erinyes, female spirits who punished wrongdoing and were later identified with the Roman Furies, were called the Eumenides, "the well-minded ones." Etymonline lists Euxine, the Greek name for the Black Sea, as another euphemistic name. A source it quotes says that Athenians referred to prison as "the chamber."

English took the word in the 1650s as a technical term of rhetoric. It named a figure of speech, not a social habit. Etymonline dates the broader modern sense to 1793: choosing a milder word or phrase than the one actually meant.

Its opposite arrived much later. Dysphemism, swapping in a harsher or more insulting word for a neutral one, dates from 1873. It uses the same pheme with dys-, "bad."

Euphemisms also wear out. In 1974 the linguist Sharon Henderson Taylor described a "euphemism cycle." Steven Pinker called it "the euphemism treadmill" in a 1994 New York Times essay, "The Game of the Name." A polite word slowly absorbs the stigma of the thing it names, so it needs replacing in turn. Wikipedia traces the toilet through this cycle. Privy-house and bog-house gave way to house-of-office, then toilet. In the 20th century came bathroom, washroom, restroom and powder room.

Each new term starts out as speaking well. It ends up as the plain word, and the search for good omens begins again.