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]