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.
       [1913 Webster]

             He that wrestles with us strengthens our nerves and
             sharpens our skill.                  --Burke.
       [1913 Webster]
   (c) To make more eager; as, to sharpen men's desires.
       [1913 Webster]

             Epicurean cooks
             Sharpen with cloyless sauce his appetite. --Shak.
       [1913 Webster]
   (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.
       [1913 Webster]

             Inclosures not only preserve sound, but increase
             and sharpen it.                      --Bacon.
       [1913 Webster]
   (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.
       [1913 Webster]

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]

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "sharpen":
   accelerate, aculeate, acuminate, agent provocateur, aggravate,
   amplify, animate, annoy, augment, barb, beef up, blow up, build up,
   complicate, concentrate, condense, consolidate, cultivate,
   cuspidate, deepen, deteriorate, double, dress, edge, embitter,
   enhance, enlarge, enliven, exacerbate, exaggerate, exasperate,
   excite, exhilarate, file, fillip, galvanize, grind, heat up,
   heighten, hone, hop up, hot up, increase, infuse life into,
   instrument, intensify, invigorate, irritate, jazz up, key up,
   machine, magnify, make acute, make complex, make sensitive,
   make worse, mechanize, mill, motorize, oilstone, pick up, pique,
   point, provoke, quicken, ramify, redouble, refine, reinforce,
   renew, reset, resuscitate, retool, revive, sensibilize, sensitize,
   set, sharp, soup up, sour, spiculate, spur, step up, stimulate,
   stir, strap, strengthen, stroke, strop, taper, tool, triple, whet,
   worsen