Origins
Joy is one of the oldest and most common emotion-words in English, borrowed from Old French joie around 1200 and ultimately descended from Latin gaudia, the neuter plural of gaudium ("delight"), which French reanalysed as a feminine singular. Its deeper ancestor is the Proto-Indo-European root *geh₂w- ("to rejoice"), which also surfaces in Greek gaio ("I rejoice") and possibly in the archaic Latin verb gaudere ("to be glad"). The word is striking for three reasons: it is a Latin plural hiding in plain sight, it is one of the many emotional Romance imports that pushed aside native Old English vocabulary after the Norman Conquest, and its family in English includes words as stylistically opposed as rejoice and gaudy.
The word joy enters English from Old French joie, from Latin gaudia (joys, the plural of gaudium), from the verb gaudere (to rejoice, to be glad), from PIE *geh₂w- (to rejoice). The earliest English attestations cluster around 1200, in the Ancrene Riwle and in early Middle English devotional prose, where it competes with and gradually displaces the native Old English wordbliss and wynn. By the time of Chaucer in the 1380s, joy is unmarked vernacular: the Canterbury Tales use it freely. The Authorised Version of 1611 made the word canonical in English religious registers — "enter thou into the joy of thy lord" — and by the eighteenth century it had fully naturalised.
The phonological transformation from Latin /g/ to French /dʒ/ (and then English /dʒ/) is a regular feature of the Gallo-Romance sound changes that shaped French from Latin. Before front vowels /e/ and /i/, Latin /g/ palatalised first to /dʒ/ and later, in modern French, to /ʒ/. The same shift produced jardin from a Frankish source, gentil from Latin gentilis, and genre from genus. English, borrowing French at the Anglo-Norman stage when /dʒ/ was still current, preserved the affricate — which is why English joy sounds like its French ancestor of 1100 rather than its French cousin of today.
Middle English
The compound enjoy (en- + joy, "to put into a state of joy") and the verb rejoice (re- + Old French joir, "to be glad," "to joy again") both derive from the same root. Rejoice is first attested around 1300 in the sense of "to gladden"; its intransitive sense ("to feel joy") emerges in the 14th century. Enjoy is first attested around 1380 with the sense "to put into a state of joy," narrowing by the 16th century to the modern meaning "to take pleasure in." Gaudy may also belong to this family — it originally meant "festive" or "joyfully ornamented" (from Latin gaudium) before deteriorating in the 16th century to mean "tastelessly bright." The rare English gaud ("a showy ornament, a bauble") preserves the older festive sense.
Across the Romance languages the Latin gaudium produced a rich family: French joie, Italian gioia, Spanish gozo and joya (the last now meaning "jewel," a semantic drift from "precious joy" to "precious object" that also happened in French joyau). Catalan goig still survives in religious contexts. Portuguese gozo means both "enjoyment" and, in modern vulgar usage, "orgasm" — a drift toward bodily pleasure that parallels Spanish gozo. Italian gioia has held the purest semantic line, meaning "joy" as an emotion and also, by metaphor, "jewel," the same double sense as Spanish joya.
The Latin gaudere appears in the opening of the Catholic hymn Gaudeamus igitur ("Let us therefore rejoice"), sung at European university ceremonies since at least the medieval period and set to its familiar melody in the eighteenth century. The imperative gaude ("rejoice!") is preserved in the English phrase "to make a gaudy" (to hold a festive celebration), still used at some Oxford colleges for annual alumni dinners. The Magnificat of Mary — "et exultavit spiritus meus in Deo salutari meo" — uses exultavit for "rejoiced," but the medieval antiphons surrounding it are saturated with gaude and gaudium, which is why the word accumulated such religious weight.
Proto-Indo-European Roots
The PIE root *geh₂w- is less productive than some, but it appears securely in Greek gaio ("I rejoice") and in Tocharian kaw- ("to love, desire"). The Germanic side shows fewer clear descendants; the Old English bliss (from *blithi-, "gentle, kind") and wynn (from *wunjo, "joy, delight") belong to separate Germanic root-families. This is why the Norman Conquest shifted the emotional vocabulary of English so decisively: the Romance imports joy, pleasure, delight, rejoice, and enjoy arrived without native competitors from the same PIE root, and they slowly pushed the Old English emotion-words into archaism or into poetry.
In modern English usage, joy occupies a slightly elevated register — a step more serious than "happiness," a step warmer than "pleasure." It is the word of religious jubilation, of named emotions in lists, of Marie Kondo's question "does it spark joy," and of common personal names (Joy, Joyce, the latter via a different French route from joiuse, "joyous"). Joyful and joyous are near-synonyms but diverge subtly: joyful describes the subject who feels joy, joyous describes the occasion or thing that produces it. "Joy to the World" has kept the word high in register through four centuries of carol-singing. The word is unlikely to fade; its plural Latin origin is a quiet curiosity that most speakers never meet.