The main meal, not a fixed hour
In present-day English, dinner is the day’s chief meal. For many speakers that sitting now falls in the evening. In other communities, especially in parts of Britain and Ireland, dinner can still mean a substantial meal at midday, with tea or supper later. The same noun also names a formal banquet and, more loosely, the food prepared for any of those occasions. School dinner, Sunday dinner, and Christmas dinner keep the older daytime timetable even where weekday dinner has moved to the evening. What the word has marked in English from the beginning is rank on the day’s table, not a clock time.
A French infinitive that became an English noun
The origin of the word dinner is a borrowing from Old French, not a native Germanic inheritance. Around 1300, Middle English diner, also spelled dyner, named the first large meal of the day, typically eaten between about nine in the morning and noon. The source was Old French disner, recorded as a verb in the eleventh century meaning to take the first meal, and then used as a noun. Modern French keeps the descendant dîner for both the verb and the meal.
Old English already had words for eating and for meals. Dinner arrived with Norman French after the Conquest and settled in Middle English as the ordinary name for the principal sitting. The related verb dine comes from the same Old French verb. The extra n in dinner, against the single n in dine, is an English spelling habit after a short vowel. It is not a separate suffix with a meaning of its own.
How a fast was undone
The French verb is a Romance formation. Historical dictionaries reconstruct a Gallo-Romance or Vulgar Latin verb along the lines of desiunare, disiūnāre, or disieiunare, built to mean breaking a fast. The pieces are Latin dis-, a prefix that reverses an action, and Late Latin ieiunare or jejunare, to fast, from classical ieiunus, fasting or hungry. The reconstructions do not match letter for letter, because the spoken form was never a standard written Latin word. The analysis of the parts is stable across the sources.
That analysis is the same idea English later spelled out in native pieces as breakfast. The two English nouns are not cognates. Breakfast is a transparent English compound. Dinner is a French loan whose Latin anatomy happens to mean the same thing: to undo a fast.
French later split the inherited stock into a doublet. Déjeuner, from the same Vulgar Latin verb, took over the earlier slot, breakfast in older usage and lunch in much of modern France, while dîner moved later in the day. Spanish desayuno and Portuguese desjejum show the same reversal-plus-fast pattern for the morning meal. Romanian dejun likewise keeps a sense closer to a midday or morning sitting. Italian digiunare, to fast, preserves the Latin fasting verb rather than the compound that undoes it.
English jejune, now usually thin or unsatisfying in a figurative sense, is a learned borrowing from the same Latin adjective ieiunus. It is a distant cousin of dinner, not a stage on the word’s path into English.
The sitting that kept moving later
English has used dinner for the chief meal from the start. What changed is when people sat down to it. In medieval Europe the main sitting was around midday, and in earlier practice it could be as early as nine or ten in the morning. That is why Middle English diner could overlap with what speakers now call breakfast, and why older English also records a sense morning meal beside the more common first big meal.
Through the eighteenth century, midday dinner remained ordinary for much of Europe. Fashion pushed the hour later. In France under the old regime, two or three in the afternoon was typical; after the revolutionary assembly sat into the late afternoon, dining drifted toward six or later, except in the countryside. English court hours followed a similar creep: four o’clock under George III, then six and seven under George IV, then eight in the Victorian court. A mid-afternoon dinette was even prescribed as a medical compromise when the fashionable hour seemed too late. Urban and courtly classes led the shift. Rural and working schedules held to an earlier dinner far longer, which is one reason British usage still splits along class and region.
English therefore still disagrees with itself. In some British speech, dinner is lunchtime and the evening meal is tea. In much of North America, dinner is the evening meal and lunch occupies the middle of the day. Supper, from Old French super and related to soup, named a lighter evening repast when dinner was still a daytime event. Where dinner moved to the evening, supper and dinner began to overlap, and the later social distinction between a formal dinner and an informal supper is an overlay on that older timetable, not the original contrast.
The word did not change its core job. The day’s main meal moved, and the name went with it.
The first sitting still showing through
If one fact is hidden by the modern evening table, it is this: dinner is historically the meal that ended a fast, and French dîner and French déjeuner are doublets from that same break-the-fast verb. English kept the older loan for the main meal and coined breakfast for the first one. An evening dinner is a late custom. The word itself is a medieval name for an earlier sitting that has been walking forward through the day for seven centuries.