brunch

"first meal of the day," mid-15c., from the verbal phrase; see break (v.) + fast (n.). For vowel shift, see below. An Old English word for it was undernmete (see undern), also morgenmete "morning meal."

Spanish almuerzo "lunch," but formerly and still locally "breakfast," is from Latin admorsus, past participle of admordere "to bite into," from ad "to" + mordēre "to bite" (see mordant). German Frühstück is from Middle High German vruostücke, literally "early bit."

In common with almuerzo, words for "breakfast" tend over time to shift in meaning toward "lunch;" compare French déjeuner "breakfast," later "lunch" (cognate of Spanish desayuno "breakfast"), from Vulgar Latin *disieiunare "to breakfast," from Latin dis- "apart, in a different direction from" + ieiunare, jejunare "fast" (see jejune; also compare dine). Greek ariston in Homer and Herodotus was a meal at the break of day but in classical times taken in the afternoon.

The long/short vowel contrast in break/breakfast represents a common pattern where words from Old English have a long vowel in their modern form but a short vowel as the first element of a compound: Christ/Christmas, holy/holiday, moon/Monday, sheep/shepherd, wild/wilderness, etc.

"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."