The Collaborative International Dictionary of English v.0.48:

Fortify \For"ti*fy\, v. i.
   To raise defensive works. --Milton.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fortify \For"ti*fy\, v. t. [imp. & p. p. Fortified; p. pr. &
   vb. n. Fortifying.] [F. fortifier, L. fortificare; fortis
   strong + -ficare (in comp.) to make. See Fort, and -fy.]
   1. To add strength to; to strengthen; to confirm; to furnish
      with power to resist attack.
      [1913 Webster]

            Timidity was fortified by pride.      --Gibbon.
      [1913 Webster]

            Pride came to the aid of fancy, and both combined to
            fortify his resolution.               --Sir W.
                                                  Scott.
      [1913 Webster]

   2. To strengthen and secure by forts or batteries, or by
      surrounding with a wall or ditch or other military works;
      to render defensible against an attack by hostile forces.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fortify
    v 1: make strong or stronger; "This exercise will strengthen
         your upper body"; "strengthen the relations between the two
         countries" [syn: strengthen, beef up, fortify] [ant:
         weaken]
    2: enclose by or as if by a fortification [syn: fortify,
       fort]
    3: prepare oneself for a military confrontation; "The U.S. is
       girding for a conflict in the Middle East"; "troops are
       building up on the Iraqi border" [syn: arm, build up,
       fortify, gird] [ant: demilitarise, demilitarize,
       disarm]
    4: add nutrients to; "fortified milk"
    5: add alcohol to (beverages); "the punch is spiked!" [syn:
       spike, lace, fortify]

Moby Thesaurus II by Grady Ward, 1.0:

126 Moby Thesaurus words for "fortify":
   add to, adulterate, affirm, animate, arm, armor, armor-plate,
   arouse, attest, augment, authenticate, back, back up, bank,
   barricade, bastardize, battle, bear out, beef up, blockade,
   bolster, boost, brace, brace up, brisk, brisken, buck up, bulwark,
   buoy, buttress, case harden, castellate, certify, cheer, chirk up,
   circumstantiate, confirm, contaminate, corroborate, corrupt,
   crenellate, cut, debase, denaturalize, denature, dig in, dilute,
   doctor, doctor up, document, embattle, embolden, encourage,
   energize, enhance, enliven, enrich, entrench, exhilarate, fence,
   fresh up, freshen, freshen up, garrison, gird, harden, hearten,
   increase, invigorate, lace, man, man the garrison, mine, nerve,
   palisade, perk up, pick up, pollute, prepare, probate, prop, prove,
   quicken, rally, ratify, ready, reanimate, reassure, recreate,
   recruit, refresh, refreshen, regale, reinforce, reinvigorate,
   renew, restore, restrengthen, revive, revivify, rouse, secure,
   set up, shore up, spike, steel, stiffen, stimulate, stir,
   strengthen, substantiate, supplement, support, sustain,
   tamper with, temper, toughen, undergird, uphold, validate, verify,
   vivify, wall, warrant, water, water down