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]

WordNet (r) 3.0 (2006):

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, underpin, bear out,
       support]