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

The Collaborative International Dictionary of English v.0.48:

Strengthening \Strength"en*ing\, a.
   That strengthens; giving or increasing strength. --
   Strength"en*ing*ly, adv.
   [1913 Webster]

   Strengthening plaster (Med.), a plaster containing iron,
      and supposed to have tonic effects.
      [1913 Webster]

WordNet (r) 3.0 (2006):

strengthening
    n 1: becoming stronger [ant: weakening]
    2: the act of increasing the strength of something [ant:
       weakening]