breakfast (n.) Look up breakfast at Dictionary.com
mid-15c., from break (v.) + fast (n.). For vowel shift, see met (v.). An Old English word for it was undernmete (see undern), also morgenmete "morning meal." The verb is recorded from 1670s. Related: Breakfasted; breakfasting.

Spanish almuerzo "lunch," but formerly and still locally "breakfast," is from Latin admorsus, past participle of admordere "to bite into," from ad "to" + mordere "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.