nuncheon (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).

"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."