What does "undern" mean?

Undern: 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.