jejune (adj.)

"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.

c. 1300, dinen, "eat the chief meal of the day, take dinner;" also in a general sense "to eat," from Old French disner  "to dine, eat, have a meal" (Modern French dîner), originally "take the first meal of the day," from stem of Gallo-Roman *desjunare "to break one's fast," from Vulgar Latin *disjejunare, from dis- "undo, do the opposite of" (see dis-) + Late Latin jejunare "to fast," from Latin iejunus "fasting, hungry, not partaking of food" (see jejune).

Transitive sense of "give a dinner to" is from late 14c. To dine out "take dinner away from home" is by 1758.