The Collaborative International Dictionary of English v.0.48:

Shrewd \Shrewd\, a. [Compar. Shrewder; superl. Shrewdest.]
   [Originally the p. p. of shrew, v.t.]
   1. Inclining to shrew; disposing to curse or scold; hence,
      vicious; malicious; evil; wicked; mischievous; vexatious;
      rough; unfair; shrewish. [Obs.] --Chaucer.
      [1913 Webster]

            [Egypt] hath many shrewd havens, because of the
            great rocks that ben strong and dangerous to pass
            by.                                   --Sir J.
                                                  Mandeville.
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            Every of this happy number
            That have endured shrewd days and nights with us.
                                                  --Shak.
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   2. Artful; wily; cunning; arch.
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            These women are shrewd tempters with their tongues.
                                                  --Shak.
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   3. Able or clever in practical affairs; sharp in business;
      astute; sharp-witted; sagacious; keen; as, a shrewd
      observer; a shrewd design; a shrewd reply.
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            Professing to despise the ill opinion of mankind
            creates a shrewd suspicion that we have deserved it.
                                                  --Secker.
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   Syn: Keen; critical; subtle; artful; astute; sagacious;
        discerning; acute; penetrating.

   Usage: Shrewd, Sagacious. One who is shrewd is keen to
          detect errors, to penetrate disguises, to foresee and
          guard against the selfishness of others. Shrewd is a
          word of less dignity than sagacious, which implies a
          comprehensive as well as penetrating mind, whereas
          shrewd does not.
          [1913 Webster] -- Shrewd"ly, adv. -- Shrewd"ness,
          n.
          [1913 Webster]

WordNet (r) 3.0 (2006):

shrewd
    adj 1: marked by practical hardheaded intelligence; "a smart
           businessman"; "an astute tenant always reads the small
           print in a lease"; "he was too shrewd to go along with
           them on a road that could lead only to their overthrow"
           [syn: astute, sharp, shrewd]
    2: used of persons; "the most calculating and selfish men in the
       community" [syn: calculating, calculative, conniving,
       scheming, shrewd]

Moby Thesaurus II by Grady Ward, 1.0:

95 Moby Thesaurus words for "shrewd":
   Byzantine, Daedalian, Machiavellian, Machiavellic, acute,
   all-knowing, apperceptive, appercipient, apprehending,
   apprehensive, arch, artful, astute, aware, cagey, calculated,
   calculating, canny, clever, cognizant, comprehending, conscious,
   crafty, cunning, cute, deceitful, deep, deep-laid, designing,
   devious, diplomatic, discerning, farsighted, feline, foresighted,
   foxy, guileful, heady, ingenious, insidious, insightful,
   intelligent, inventive, judicious, keen, knowing, knowledgeable,
   manipulative, mindful, omniscient, pawky, penetrating, perceptive,
   percipient, perspicacious, piercing, polite, politic, prehensile,
   probing, prudent, quick-witted, ready, resourceful, sagacious,
   sage, savvy, scheming, sensible, serpentine, sharp, shifty, slick,
   slippery, sly, smart, smooth, snaky, sneaky, sophistical, stealthy,
   strategic, subtile, subtle, supple, tactical, tidy, trickish,
   tricksy, tricky, understanding, vulpine, wary, wily, wise