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]