The Collaborative International Dictionary of English v.0.48:

Strengthen \Strength"en\, v. i.
   To grow strong or stronger.
   [1913 Webster]

         The young disease, that must subdue at length,
         Grows with his growth, and strengthens with his
         strength.                                --Pope.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Strengthen \Strength"en\, v. t. [imp. & p. p. Strengthened; p.
   pr. & vb. n. Strengthening.]
   1. To make strong or stronger; to add strength to; as, to
      strengthen a limb, a bridge, an army; to strengthen an
      obligation; to strengthen authority.
      [1913 Webster]

            Let noble Warwick, Cobham, and the rest, . . .
            With powerful policy strengthen themselves. --Shak.
      [1913 Webster]

   2. To animate; to encourage; to fix in resolution.
      [1913 Webster]

            Charge Joshua, and encourage him, and strengthen
            him.                                  --Deut. iii.
                                                  28.
      [1913 Webster]

   Syn: To invigorate; confirm; establish; fortify; animate;
        encourage.
        [1913 Webster]

WordNet (r) 3.0 (2006):

strengthen
    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: gain strength; "His body strengthened" [ant: weaken]
    3: give a healthy elasticity to; "Let's tone our muscles" [syn:
       tone, tone up, strengthen]

Moby Thesaurus II by Grady Ward, 1.0:

131 Moby Thesaurus words for "strengthen":
   accelerate, accrue, accumulate, add to, advance, affirm, aggravate,
   animate, anneal, appreciate, attest, augment, authenticate, back,
   back up, balloon, be tough, bear out, beef up, bloat, blow up,
   bolster, boom, boost, brace, brace up, breed, broaden, buttress,
   case harden, certify, cheer, chirk up, circumstantiate, complicate,
   concentrate, condense, confirm, consolidate, corroborate,
   crescendo, deepen, develop, document, double, embolden, encourage,
   endure, energize, enhance, exacerbate, exaggerate, fortify, gain,
   gain strength, gelt, get ahead, gird, go up, grow, hang tough,
   harden, hearten, heat up, heighten, hop up, hot up, increase,
   inspirit, intensify, invigorate, jazz up, key up, magnify,
   make complex, mount, multiply, nerve, nourish, nutrify, prepare,
   probate, proliferate, prop, prove, ramify, ratify, ready, recruit,
   redouble, refresh, reinforce, reinvigorate, rejuvenate, renew,
   restrengthen, rigidify, rise, run up, screw up, sharpen, shoot up,
   shore up, sinew, snowball, soup up, spread, steel, step up,
   stiffen, substantiate, supplement, support, sustain, swell, temper,
   tense, tighten, tone up, toughen, trice up, triple, undergird,
   uphold, validate, verify, vitalize, warrant, wax, whet, widen