The Collaborative International Dictionary of English v.0.48:

Underpinning \Un"der*pin`ning\, n.
   1. The act of one who underpins; the act of supporting by
      stones, masonry, or the like.
      [1913 Webster]

   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.]
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Underpin \Un`der*pin"\, v. t. [imp. & p. p. Underpinned; p.
   pr. & vb. n. Underpinning.]
   1. To lay stones, masonry, etc., under, as the sills of a
      building, on which it is to rest.
      [1913 Webster]

   2. To support by some solid foundation; to place something
      underneath for support.
      [1913 Webster]