The Collaborative International Dictionary of English v.0.48:

Undergird \Un`der*gird"\, v. t.
   To bind below; to gird round the bottom.
   [1913 Webster]

         They used helps, undergirding the ship.  --Acts xxvii.
                                                  17.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "undergird":
   affirm, afford support, attest, authenticate, back, back up, bear,
   bear out, bear up, bed on, beef up, bolster, bolster up, bottom on,
   brace, brace up, build on, buoy up, buttress, carry, case harden,
   certify, circumstantiate, confirm, corroborate, cradle, crutch,
   cushion, document, finance, fortify, found on, fund, gird,
   give support, ground on, harden, hold, hold up, invigorate, keep,
   keep afloat, keep up, lend support, mainstay, maintain, nerve,
   pillow, probate, prop, prop up, prove, ratify, refresh, reinforce,
   reinvigorate, restrengthen, set on, shore, shore up, shoulder,
   stay, steel, stiffen, strengthen, subsidize, substantiate,
   subvention, subventionize, support, sustain, temper, toughen,
   underbrace, underlie, underpin, underset, upbear, uphold, upkeep,
   validate, verify, warrant