\Un"der*pin`ning\, n.
1. The act of one who underpins; the act of supporting by
   stones, masonry, or the like.
2. (Arch.)
   (a) That by which a building is underpinned; the material
       and construction used for support, introduced beneath
       a wall already constructed.
   (b) The foundation, esp. of a frame house. [Local, U. S.]