"first meal of the day," mid-15c., from the verbal phrase; see break (v.) + fast (n.). For vowel shift, see met (v.). 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.
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]
proper name of a coffee substitute, 1895, from a Latinized form of the name of American manufactured foods pioneer Charles William Post (1854-1914), founder of the breakfast cereal company.
upbeat parting exclamation, British, 1896 as cheero; 1918 as cheerio; from cheer. The breakfast cereal Cheerios debuted in 1941 as CheeriOats; the name was shortened in 1945.