fay (n.)

late 14c., "one's lot or destiny; predetermined course of life;" also "one's guiding spirit," from Old French fate and directly from Latin fata (source also of Spanish hado, Portuguese fado, Italian fato), neuter plural of fatum "prophetic declaration of what must be, oracle, prediction," thus the Latin word's usual sense, "that which is ordained, destiny, fate," literally "thing spoken (by the gods)," from neuter past participle of fari "to speak," from PIE root *bha- (2) "to speak, tell, say." Often in a bad sense in Latin: "bad luck, ill fortune; mishap, ruin; a pest or plague."

From early 15c. as "power that rules destinies, agency which predetermines events; supernatural predetermination;" also "destiny personified." Meaning "that which must be" is from 1660s; sense of "final event" is from 1768. The Latin sense evolution is from "sentence of the Gods" (Greek theosphaton) to "lot, portion" (Greek moira, personified as a goddess in Homer).

The sense of "one of the three goddesses (Clotho, Lachesis, and Atropos) who determined the course of a human life" (or, as Blount has it, "the three Ladies of destiny") is in English by 1580s. Their Greek name was Moirai (see above), from a verb meaning "to receive one's share." Latin Parca "one of the three Fates or goddesses of fate" (source of French parque "a Fate;" Spanish parca "Death personified; the Grim Reaper") might be from parcere "act sparingly, refrain from; have mercy upon, forbear to injure or punish" (if so, probably here a euphemism) or plectere "to weave, plait." The native word in English was wyrd (see weird).

J'y suivais un serpent qui venait de me mordre

Quel repli de désirs, sa traîne!...Quel désordre

De trésors s'arrachant à mon avidité,

Et quelle sombre soif de la limpidité!

[Paul Valéry, from La Jeune Parque]

*bhā-; Proto-Indo-European root meaning "to speak, tell, say."

It forms all or part of: abandon; affable; anthem; antiphon; aphasia; aphonia; aphonic; apophasis; apophatic; ban (n.1) "proclamation or edict;" ban (v.); banal; bandit; banish; banlieue; banns (n.); bifarious; blame; blaspheme; blasphemy; boon (n.); cacophony; confess; contraband; defame; dysphemism; euphemism; euphony; fable; fabulous; fado; fairy; fame; famous; fandango; fatal; fate; fateful; fatuous; fay; gramophone; heterophemy; homophone; ineffable; infamous; infamy; infant; infantile; infantry; mauvais; megaphone; microphone; monophonic; nefandous; nefarious; phatic; -phone; phone (n.2) "elementary sound of a spoken language;" phoneme; phonetic; phonic; phonics; phono-; pheme; -phemia; Polyphemus; polyphony; preface; profess; profession; professional; professor; prophecy; prophet; prophetic; quadraphonic; symphony; telephone; xylophone.

It is the hypothetical source of/evidence for its existence is provided by: Greek pheme "speech, voice, utterance, a speaking, talk," phōnē "voice, sound" of a human or animal, also "tone, voice, pronunciation, speech," phanai "to speak;" Sanskrit bhanati "speaks;" Latin fari "to say," fabula "narrative, account, tale, story," fama "talk, rumor, report; reputation, public opinion; renown, reputation;" Armenian ban, bay "word, term;" Old Church Slavonic bajati "to talk, tell;" Old English boian "to boast," ben "prayer, request;" Old Irish bann "law."

c. 1300, fairie, "the country or home of supernatural or legendary creatures; fairyland," also "something incredible or fictitious," from Old French faerie "land of fairies, meeting of fairies; enchantment, magic, witchcraft, sorcery" (12c.), from fae "fay," from Latin fata "the Fates," plural of fatum "that which is ordained; destiny, fate," from PIE root *bha- (2) "to speak, tell, say." Also compare fate (n.), also fay.

In ordinary use an elf differs from a fairy only in generally seeming young, and being more often mischievous. [Century Dictionary]

But that was before Tolkien. As a type of supernatural being from late 14c. [contra Tolkien; for example "This maketh that ther been no fairyes" in "Wife of Bath's Tale"], perhaps via intermediate forms such as fairie knight "supernatural or legendary knight" (c. 1300), as in Spenser, where faeries are heroic and human-sized. As a name for the diminutive winged beings in children's stories from early 17c.

Yet I suspect that this flower-and-butterfly minuteness was also a product of "rationalization," which transformed the glamour of Elfland into mere finesse, and invisibility into a fragility that could hide in a cowslip or shrink behind a blade of grass. It seems to become fashionable soon after the great voyages had begun to make the world seem too narrow to hold both men and elves; when the magic land of Hy Breasail in the West had become the mere Brazils, the land of red-dye-wood. [J.R.R. Tolkien, "On Fairy-Stories," 1947]

Hence, figurative adjective use in reference to lightness, fineness, delicacy. Slang meaning "effeminate male homosexual" is recorded by 1895. Fairy ring, of certain fungi in grass fields (as we would explain it now), is from 1590s. Fairy godmother attested from 1820. Fossil Cretaceous sea urchins found on the English downlands were called fairy loaves, and a book from 1787 reports that "country people" in England called the stones of the old Roman roads fairy pavements.