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.

Strengthening \Strength"en*ing\, a.
   That strengthens; giving or increasing strength. --
   {Strength"en*ing*ly}, adv.

   {Strengthening plaster} (Med.), a plaster containing iron,
      and supposed to have tonic effects.

Source : WordNet®

strengthening
     adj : invigorating physically or mentally [syn: {fortifying}]
     n 1: becoming stronger [ant: {weakening}]
     2: the act of increasing the strength of something [ant: {weakening}]