Source : Webster's Revised Unabridged Dictionary (1913)

Underpinning \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.]

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.

   2. To support by some solid foundation; to place something
      underneath for support.

Source : WordNet®

underpinning
     See {underpin}

underpin
     v 1: support from beneath
     2: support with evidence or authority or make more certain or
        confirm; "The stories and claims were born out by the
        evidence" [syn: {corroborate}, {bear out}, {support}]
     [also: {underpinning}, {underpinned}]