c. 1300, "first big meal of the day" (eaten between 9 a.m. and noon), from Old French disner "breakfast" (11c.), noun use of infinitive disner (Modern French dîner) "take the first meal of the day," from stem of Gallo-Roman *desjunare "to break one's fast," from Vulgar Latin *disjejunare, from dis- "undo, do the opposite of" (see dis-) + Late Latin jejunare "to fast," from Latin iejunus "fasting, hungry, not partaking of food" (see jejune).
Always used in English for the main meal of the day, but the time of that has gradually shifted later.
In medieval and modern Europe the common practice, down to the middle of the eighteenth century, was to take this meal about midday, or in more primitive times even as early as 9 or 10 A.M. In France, under the old régime, the dinner-hour was at 2 or 3 in the afternoon; but when the Constituent Assembly moved to Paris, since it sat until 4 or 5 o'clock, the hour for dining was postponed. The custom of dining at 6 o'clock or later has since become common, except in the country, where early dinner is still the general practice. [Century Dictionary, 1897]
The change from midday to evening began with the fashionable classes. Compare dinette.
Dinner-time is attested from late 14c.; dinner-hour is from 1750. Dinner-table is from 1784; dinner-jacket from 1852; dinner-party by 1780. Childish reduplication din-din is attested from 1905.