1. To make strong or stronger; to add strength to; to increase the strength of; to fortify; to reinforce.
    to strengthen a limb, a bridge, an army
    to strengthen an obligation
    to strengthen authority
  2. To animate; to give moral strength to; to encourage; to fix in resolution; to hearten.
  3. To augment; to improve; to intensify.
  4. To grow strong or stronger.

The above text is a snippet from Wiktionary: strengthen
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