"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?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."
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]
Insane; crazy; eccentric: On critical issues of fact and analysis he is out to lunch (1955+ Students)
eat someone's lunch, out to lunch, shoot one's cookies