Etymology
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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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forenoon (n.)
"the morning," especially the latter part of it, when business is done, c. 1500, from fore- + noon.
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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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afternoon (n.)

"part of the day from noon to evening," c. 1300, from after + noon. In 15c.-16c., the form was at afternoon; from c. 1600 it has been in the afternoon. As an adjective from 1570s. Middle English had also aftermete "afternoon, part of the day following the noon meal" (mid-14c.).

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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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luncheon (n.)
"light repast between mealtimes," 1650s (lunching; spelling luncheon by 1706); earlier "thick piece, hunk (of bread)," 1570s (luncheon), which is of uncertain origin. Perhaps it is based on northern English dialectal lunch "hunk of bread or cheese" (1580s; said to be probably from Spanish lonja "a slice," literally "loin"), blended with or influenced by nuncheon (Middle English nonechenche, mid-14c.) "light mid-day meal," from none "noon" (see noon) + schench "drink," from Old English scenc, from scencan "pour out."

Despite the form lunching in the 1650s source OED discounts that it possibly could be from lunch (v.), which is first attested more than a century later. It suggests perhaps an analogy with truncheon, etc., or to simulate a French origin. Especially in reference to an early afternoon meal eaten by those who have a noontime dinner.
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nones (n.)

late 14c. in reference to the Roman calendar, "ninth day (by inclusive reckoning) before the ides of each month" (7th of March, May, July, October, 5th of other months), from Old French nones and directly from Latin nonæ (accusative nonas), fem. plural of nonus "ninth," contracted from *novenos, from novem "nine" (see nine).

The ecclesiastical sense of "daily office said originally at the ninth hour of the day" is from 1709; originally fixed at ninth hour from sunrise, hence about 3 p.m. (now usually somewhat earlier), from Latin nona (hora) "ninth (hour)," from fem. plural of nonus "ninth." It was also used from c. 1300 in a general sense of "midday" (see noon). For for the nones, see nonce.

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lunch (n.)

"mid-day repast, small meal between breakfast and dinner," 1786, a shortened form of luncheon (q.v.) in this sense (1650s), which is of uncertain origin; it appears to be identical with an older word meaning "thick piece, hunk" (1570s), which perhaps evolved from lump (n.) [OED]. There also was a contemporary nuncheon "light mid-day meal," from noon + Middle English schench "drink." Old English had nonmete "afternoon meal," literally "noon-meat" (Middle English non-mete). The verb meaning "to take to lunch" (said to be from the noun) also is attested from 1786:

PRATTLE. I always to be ſure, makes a point to keep up the dignity of the family I lives in. Wou'd you take a more ſolid refreſhment?—Have you lunch'd, Mr. Bribe?
BRIBE. Lunch'd O dear! Permit me, my dear Mrs. Prattle, to refreſh my sponge, upon the honey dew that clings to your raviſhing pouters. O! Mrs. Prattle, this ſhall be my lunch. (kiſſes)
["The Mode," in William Davies' "Plays Written for a Private Theatre," London, 1786]

As late as 1817 the only definition of lunch (n.) in Webster's is "a large piece of food," but this is now obsolete or provincial. OED says in 1820s the word "was regarded either as a vulgarism or as a fashionable affectation." Related: Lunched; lunching.

Lunch money is attested from 1868. Lunch-time is from 1821; lunch hour is from 1840; lunch-break is from 1960. Slang phrase out to lunch "insane, stupid, clueless" first recorded 1955, on notion of being "not there."

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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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Yugoslav (n.)
1853, from Slav + Serbo-Croatian jugo- "south," combining form of jug "south, south wind, noon," from Old Church Slavonic jugu "south, south wind, noon."
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