What does "undermeal" mean?

Undermeal: Hence, something occurring or done in the afternoon; esp., an afternoon meal; supper; also, an afternoon nap; a siesta. [Obs. or Prov. Eng.] Another great supper, or undermeal, was made ready for them, coming home from ditching and plowing. Withals (1608). I think I am furnished with Cattern [Catharine] pears for one undermeal. B. Jonson. In a narrower limit than the forty years' undermeal of the seven sleepers. Nash.