What is "undern"

Undern \Un"dern\, n. [AS. undern; akin to OS. undorn, OHG. untarn, untorn, Icel. undorn mid afternoon, mid forenoon, Goth. unda['u]rnimats the midday meal. Cf. Undermeal, Undertime.] The time between; the time between sunrise and noon; specifically, the third hour of the day, or nine o'clock in the morning, according to ancient reckoning; hence, mealtime, because formerly the principal meal was eaten at that hour; also, later, the afternoon; the time between dinner and supper. [Obs. or Prov. Eng.]

Betwixt undern and noon was the field all won.
--R. of Brunne.

In a bed of worts still he lay Till it was past undern of the day.
--Chaucer.

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."

n. (label en obsolete) The third ecclesiastical hour of the day, at around nine o'clock in the morning; tierce. (10^th-15^thc.)