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):

strengthened
    adj 1: given added strength or support; "reinforced concrete
           contains steel bars or metal netting" [syn: reinforced,
           strengthened]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "strengthened":
   accelerated, aggrandized, amplified, augmented, backed, beefed-up,
   bloated, boosted, broadened, calcified, callous, calloused,
   case-hardened, crusted, crusty, crystallized, deepened, elevated,
   enhanced, enlarged, expanded, extended, fossilized, granulated,
   hardened, heightened, hiked, hornified, increased, incrusted,
   indurate, indurated, inflated, intensified, jazzed up, lapidified,
   magnified, multiplied, ossified, petrified, proliferated, raised,
   reinforced, rigidified, sclerotic, set, solidified, spread,
   steeled, stiffened, swollen, tightened, toughened, vitrified,
   widened