Source : Webster's Revised Unabridged Dictionary (1913)

Underdrain \Un"der*drain`\, n.
   An underground drain or trench with openings through which
   the water may percolate from the soil or ground above.

Underdrain \Un`der*drain"\, v. t.
   To drain by forming an underdrain or underdrains in; as, to
   underdrain land.