mid-15c., from break (v.) + fast (n.). The verb is recorded from 1670s. The Sp. almuerzo "lunch," but formerly and still locally "breakfast," is from L. admorsus, pp. of admordere "to bite into," from ad- "to" + mordere "to bite." Like almuerzo,
words for "breakfast" tend over time to shift in meaning toward "lunch;" cf. Fr. déjeuner "breakfast," later "lunch" (equivalent of Sp. desayuno "breakfast"), both from V.L. *disieiunare "to breakfast," from L. dis- + ieiunare, jejunare "fast" (see jejune; also cf. dine).