Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

   2. To animate; to encourage; to fix in resolution.

            Charge Joshua, and encourage him, and strengthen
            him.                                  --Deut. iii.
                                                  28.

   Syn: To invigorate; confirm; establish; fortify; animate;
        encourage.

Strengthen \Strength"en\, v. i.
   To grow strong or stronger.

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

Source : WordNet®

strengthen
     v 1: make strong or stronger; "This exercise will strengthen your
          upper body"; "strenghten the relations between the two
          countries" [syn: {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}]