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. [
1913 Webster]
"Betwixt undern and noon was the field all won."
[1913 Webster]
"In a bed of worts still he lay
Till it was past undern of the day."
[1913 Webster]