The Collaborative International Dictionary of English v.0.48:

Sharpen \Sharp"en\, v. t. [imp. & p. p. Sarpened; p. pr. & vb.
   n. Sharpening.] [See Sharp, a.]
   To make sharp. Specifically:
   (a) To give a keen edge or fine point to; to make sharper;
       as, to sharpen an ax, or the teeth of a saw.
   (b) To render more quick or acute in perception; to make more
       ready or ingenious.
       [1913 Webster]

             The air . . . sharpened his visual ray
             To objects distant far.              --Milton.
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             He that wrestles with us strengthens our nerves and
             sharpens our skill.                  --Burke.
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   (c) To make more eager; as, to sharpen men's desires.
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             Epicurean cooks
             Sharpen with cloyless sauce his appetite. --Shak.
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   (d) To make more pungent and intense; as, to sharpen a pain
       or disease.
   (e) To make biting, sarcastic, or severe. "Sharpen each
       word." --E. Smith.
   (f) To render more shrill or piercing.
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             Inclosures not only preserve sound, but increase
             and sharpen it.                      --Bacon.
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   (g) To make more tart or acid; to make sour; as, the rays of
       the sun sharpen vinegar.
   (h) (Mus.) To raise, as a sound, by means of a sharp; to
       apply a sharp to.
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The Collaborative International Dictionary of English v.0.48:

Sharpen \Sharp"en\, v. i.
   To grow or become sharp.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sharpen
    v 1: make sharp or sharper; "sharpen the knives" [ant: blunt,
         dull]
    2: make crisp or more crisp and precise; "We had to sharpen our
       arguments"
    3: become sharp or sharper; "The debate sharpened"
    4: put (an image) into focus; "Please focus the image; we cannot
       enjoy the movie" [syn: focus, focalize, focalise,
       sharpen] [ant: blear, blur]
    5: make (images or sounds) sharp or sharper [ant: soften]
    6: raise the pitch of (musical notes) [ant: drop, flatten]
    7: give a point to; "The candles are tapered" [syn: sharpen,
       taper, point]
    8: make (one's senses) more acute; "This drug will sharpen your
       vision" [syn: sharpen, heighten]