The Collaborative International Dictionary of English v.0.48:

Freshen \Fresh"en\, v. i.
   1. To grow fresh; to lose saltness.
      [1913 Webster]

   2. To grow brisk or strong; as, the wind freshens.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Freshen \Fresh"en\, v. t. [imp. & p. p. Freshened; p. pr. &
   vb. n. Freshening]
   1. To make fresh; to separate, as water, from saline
      ingredients; to make less salty; as, to freshen water,
      fish, or flesh.
      [1913 Webster]

   2. To refresh; to revive. [Obs.] --Spenser.
      [1913 Webster]

   3. (Naut.) To relieve, as a rope, by change of place where
      friction wears it; or to renew, as the material used to
      prevent chafing; as, to freshen a hawse. -- Totten.
      [1913 Webster]

   To freshen ballast (Naut.), to shift Or restore it.

   To freshen the hawse, to pay out a little more cable, so as
      to bring the chafe on another part.

   To freshen the way, to increase the speed of a vessel.
      --Ham. Nav. Encyc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

freshen
    v 1: make (to feel) fresh; "The cool water refreshed us" [syn:
         refresh, freshen]
    2: become or make oneself fresh again; "She freshened up after
       the tennis game" [syn: freshen, refresh, refreshen,
       freshen up]
    3: make fresh again [syn: refresh, freshen, refreshen]
       [ant: fag, fag out, fatigue, jade, outwear, tire,
       tire out, wear, wear down, wear out, wear upon,
       weary]

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "freshen":
   aerate, air, air out, air-condition, air-cool, airify, animate,
   blast, bleach, blow, blow a hurricane, blow great guns, blow over,
   blow up, bluster, bowdlerize, brace, brace up, breeze, breeze up,
   brew, brighten up, brisk, brisken, brush up, buck up, cheer, chill,
   chirk up, clean, clean out, clean up, cleanse, clear out, come up,
   cool, cross-ventilate, delouse, deodorize, depurate, deterge,
   dry-clean, dust, dust off, enliven, exhilarate, expurgate, fan,
   fix, fortify, fresh up, freshen up, furbish, furbish up, gather,
   huff, ice, ice-cool, increase, infrigidate, invigorate, lace,
   lustrate, oxygenate, oxygenize, perk up, pick up, pipe up, polish,
   polish up, puff, purge, purify, quicken, rage, reanimate, recreate,
   reform, refresh, refreshen, refrigerate, regale, reinvigorate,
   renew, renovate, repair, retouch, revitalize, revive, revivify,
   rouse, rub up, scavenge, set in, set up, shine, spike, spruce,
   spruce up, squall, steam-clean, stimulate, storm, strengthen,
   sweep out, sweeten, tidy, titivate, tone up, touch up, vamp,
   vamp up, ventilate, vivify, waft, whiff, whiffle, whiten, wind,
   winnow, wipe, wipe off, wipe out, wipe up