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]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "underpinning":
   base, basement, basis, bearing wall, bed, bedding, bedrock, brace,
   buttress, column, floor, flooring, fond, footing, foundation,
   fundament, fundamental, ground, grounds, groundwork, hardpan,
   infrastructure, pavement, principle, prop, radical, riprap,
   rock bottom, root, rudiment, seat, seating, shore, sill,
   solid ground, solid rock, stay, stereobate, stylobate, substratum,
   substruction, substructure, terra firma, underbuilding,
   undercarriage, undergirding, understruction, understructure