undern (n.)

an obsolete Old English and Middle English word for "morning;" in Old English originally "third hour of the day; 9 a.m." (corresponding to tierce). Hence underngeweorc, undernmete "breakfast." Common Germanic: Old Frisian unden, Old Saxon undorn, Middle Dutch onderen, Old High German untarn, Old Norse undorn; of uncertain origin. By extension, "period from 9 a.m. to noon;" but from 13c. shifting to "midday, noon" (as in undern-mete "lunch," 14c.); and by late 15c. to "late afternoon or early evening."