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.
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.
"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." 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."
"pertaining to dinner" or other meal, 1820, from Latin prandium "late breakfast, luncheon," from *pram "early" (from PIE *pre-, variant of root *per- (1) "forward," hence "in front of, before, first") + edere "to eat" (from PIE root *ed- "to eat") + -al (1). OED reports it as "affected or jocose." Compare postprandial.
The Court dinner-hour, in the reign of George III., was at the Hanoverian hour of four o'clock. During the reign of George IV. it gradually crept up to six o'clock, and finally became steady at the Indian hour of seven, and so remained until the reign of Her Most gracious Majesty, when the formal Court dinner-hour became eight o'clock. These innovations on the national hours of meals did not meet the approval of the medical faculty, and in consequence a dinette at two o'clock was prescribed. This has ever since been the favourite Court meal, being in reality a substantial hot repast, which has exploded the old-fashioned luncheon of cold viands. [The Queen newspaper, London, quoted in Imperial Dictionary, 1883]
also post-prandial, 1820 (Coleridge), "happening, said, done, etc. after dinner," from post- "after" + Latin prandium "luncheon" (usually bread, fish, or cold meat, taken around noon), from *pram "early" (from PIE *pre-, variant of root *per- (1) "forward," hence "in front of, before, first") + edere "to eat" (from PIE root *ed- "to eat") + -al (1). "Chiefly humorous" [OED].
ACCORDING to the Lady, to be fashionable nowadays we must "brunch." Truly an excellent portmanteau word, introduced, by the way, last year, by Mr GUY BERINGER, in the now defunct Hunter's Weekly, and indicating a combined breakfast and lunch. At Oxford, however, two years ago, an important distinction was drawn. The combination-meal, when nearer the usual breakfast hour, is "brunch," and, when nearer luncheon, is "blunch." Please don't forget this. [Punch, Aug. 1, 1896]