Meaning of undercast

un•der•cast

Pronunciation: (un'dur-kast", -käst"), [key]
— n.
  1. a crossing of two passages, as airways, dug at the same level so that one descends to pass beneath the other without any opening into it. Cf.(def. 9).
  2. an overcast layer of clouds viewed from above.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.