Source : Webster's Revised Unabridged Dictionary (1913)

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.