underdrain I. \ˈ ̷ ̷ ̷ ̷ˌ ̷ ̷\noun Etymology:under (III) + drain : a concealed drain with openings through which the water enters when the water table reaches the level of the drain < an underdrain below a filter bed > II. \| ̷ ̷ ̷ ̷| ̷ ̷\transitive verb : to drain by forming an underdrain < underdrain a meadow >