Etymology
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midday (n.)

also mid-day, "the middle of the day," from Old English middæg "midday, noon," contracted from midne dæg; see mid (adj.) + day. Similar formation in Old Frisian middei, Dutch middag, Old High German mittitag, German Mittag, Old Norse miðdagr. As an adjective, "of or pertaining to midday," from early 14c.

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noonday (n.)

"middle of the day," first used by Coverdale (1535), from noon + day. As an adjective from 1650s. Old English had non tid "noon-tide, midday, noon," also non-tima "noon, noon-time, midday."

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Midi 

"southern France," 1883, from French midi "south," literally "midday" (12c.), from mi "middle" (from Latin medius "middle;" see medial (adj.)) + di "day" (from Latin dies, from PIE root *dyeu- "to shine"). At midday in the northern hemisphere the sun is in the south of the sky. Compare Latin meridianus "of midday, of noon;" also "southerly, to the south" (see meridian), and Middle English mid-dai in its secondary sense "south, to the south" (late 14c.).

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post meridiem 

"after noon, occurring after the sun has passed the meridian," applied to the time between noon and midnight, 1640s, Latin, from post "after" (see post-) + accusative of meridies "midday, noon" (see meridian).

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ante meridiem 
"of morning, before mid-day," 1560s, Latin, literally "before noon," from ante "before" (from PIE root *ant- "front, forehead," with derivatives meaning "in front of, before") + accusative of meridies "midday, noon" (see meridian). Adjective antemeridian is attested from 1650s.
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meridian (n.)

mid-14c., "noon, midday," from Old French meridien "of the noon time, midday; the meridian; a southerner" (12c.), and directly from Latin meridianus "of midday, of noon, southerly, to the south," from meridies "noon, south," from meridie "at noon," altered by dissimilation from pre-Latin *medi die, locative of medius "mid-" (from PIE root *medhyo- "middle") + dies "day" (from PIE root *dyeu- "to shine").

The cartographic sense of "a great circle or half-circle of a sphere passing through the poles" is attested from late 14c., originally astronomical. Figurative uses tend to suggest "point of highest development or fullest power," implying a subsequent decline. As an adjective from late 14c. Related: Meridional. The city in Mississippi, U.S., was settled 1854 (as Sowashee Station) at a railway junction and given its current name in 1860, supposedly by people who thought meridian meant "junction" (they perhaps confused the word with median).

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noon (n.)

mid-12c., non "midday," in exact use, "12 o'clock p.m.," also "midday meal," from Old English non "3 o'clock p.m., the ninth hour from sunrise," also "the canonical hour of nones," from Latin nona hora "ninth hour" of daylight, by Roman and ecclesiastical reckoning about 3 p.m., from nona, fem. singular of nonus "ninth," contracted from *novenos, from novem "nine" (see nine).

The sense shift from "3 p.m." to "12 p.m." began during 12c., and various reasons are given for it, such as unreliability of medieval time-keeping devices and the seasonal elasticity of the hours of daylight in northern regions. In monasteries and on holy days, fasting ended at nones, which perhaps offered another incentive to nudge it up the clock. Or perhaps the sense shift was based on an advance in the customary time of the (secular) midday meal. Whatever the cause, the meaning change from "ninth hour" to "sixth hour" seems to have been complete by 14c. (the same evolution is in Dutch noen).

From 17c. to 19c., noon sometimes also meant "midnight" (the noon of the night).

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nuncheon (n.)

mid-14c., nōn-schench, "slight refreshment of food and/or liquor taken at midday," originally taken in the afternoon, from none "noon" (see noon) + shench "draught, cup," from Old English scenc, related to scencan "to pour out, to give to drink," cognate with Old Frisian skenka "to give to drink, German, Dutch schenken "to give." Compare luncheon.

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undern (n.)
an obsolete Old English and Middle English word for "morning;" in Old English originally "third hour of the day; 9 a.m." (corresponding to tierce). Hence underngeweorc, undernmete "breakfast." Common Germanic: Old Frisian unden, Old Saxon undorn, Middle Dutch onderen, Old High German untarn, Old Norse undorn; of uncertain origin. By extension, "period from 9 a.m. to noon;" but from 13c. shifting to "midday, noon" (as in undern-mete "lunch," 14c.); and by late 15c. to "late afternoon or early evening."
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dinner (n.)

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.

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