Source : Webster's Revised Unabridged Dictionary (1913)

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®

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}]