The Collaborative International Dictionary of English v.0.48:

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.]
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         Betwixt undern and noon was the field all won. --R. of
                                                  Brunne.
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         In a bed of worts still he lay
         Till it was past undern of the day.      --Chaucer.
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