lunchyoudaoicibaDictYouDict[lunch 词源字典]
lunch: [16] When lunch first appeared on the scene, at the end of the 16th century, it was used for a ‘slice or hunk of food’ (‘He shall take bread and cut it into little lunches into a pan with cheese’, Richard Surfleet, Country Farm 1600). It appears to have been borrowed from Spanish lonja ‘slice’. The roughly contemporaneous luncheon, probably just an arbitrary lengthening of lunch, came to be used in the early 17th century for a ‘snack’ (the link with ‘hunk or piece of food’ is obvious), and eventually for a ‘light meal’. Lunch returned to the language in this sense at the beginning of the 19th century, as an abbreviation of luncheon.
[lunch etymology, lunch origin, 英语词源]
lunch (n.)youdaoicibaDictYouDict
"mid-day repast," 1786, shortened form of luncheon (q.v.). 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]
But as late as 1817 the only definition of lunch in Webster's is "a large piece of food." 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 (n.) is from 1821; lunch hour is from 1840. Slang phrase out to lunch "insane, stupid, clueless" first recorded 1955, on notion of being "not there." Old English had nonmete "afternoon meal," literally "noon-meat."