Un"dern (?), n. [AS. undern; akin to OS. undorn, OHG. untarn, untorn, Icel. undorn mid afternoon, mid forenoon, Goth. undaxa3;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.
© Webster 1913.