Source : Webster's Revised Unabridged Dictionary (1913)

Undergird \Un`der*gird"\, v. t.
   To blind below; to gird round the bottom.

         They used helps, undergirding the ship.  --Acts xxvii.
                                                  17.

Source : WordNet®

undergird
     v 1: lend moral support to
     2: make secure underneath; "undergird the ship" [syn: {brace up}]
     [also: {undergirt}]