The Collaborative International Dictionary of English v.0.48:

Sinister \Sin"is*ter\ (s[i^]n"[i^]s*t[~e]r; 277), a.

   Note: [Accented on the middle syllable by the older poets, as
         Shakespeare, Milton, Dryden.] [L. sinister: cf. F.
         sinistre.]
   1. On the left hand, or the side of the left hand; left; --
      opposed to dexter, or right. "Here on his sinister
      cheek." --Shak.
      [1913 Webster]

            My mother's blood
            Runs on the dexter cheek, and this sinister
            Bounds in my father's                 --Shak.
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   Note: In heraldy the sinister side of an escutcheon is the
         side which would be on the left of the bearer of the
         shield, and opposite the right hand of the beholder.
         [1913 Webster]

   2. Unlucky; inauspicious; disastrous; injurious; evil; -- the
      left being usually regarded as the unlucky side; as,
      sinister influences.
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            All the several ills that visit earth,
            Brought forth by night, with a sinister birth. --B.
                                                  Jonson.
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   3. Wrong, as springing from indirection or obliquity;
      perverse; dishonest; corrupt; as, sinister aims.
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            Nimble and sinister tricks and shifts. --Bacon.
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            He scorns to undermine another's interest by any
            sinister or inferior arts.            --South.
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            He read in their looks . . . sinister intentions
            directed particularly toward himself. --Sir W.
                                                  Scott.
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   4. Indicative of lurking evil or harm; boding covert danger;
      as, a sinister countenance.
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   Bar sinister. (Her.) See under Bar, n.

   Sinister aspect (Astrol.), an appearance of two planets
      happening according to the succession of the signs, as
      Saturn in Aries, and Mars in the same degree of Gemini.

   Sinister base, Sinister chief. See under Escutcheon.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sinister
    adj 1: threatening or foreshadowing evil or tragic developments;
           "a baleful look"; "forbidding thunderclouds"; "his tone
           became menacing"; "ominous rumblings of discontent";
           "sinister storm clouds"; "a sinister smile"; "his
           threatening behavior"; "ugly black clouds"; "the
           situation became ugly" [syn: baleful, forbidding,
           menacing, minacious, minatory, ominous,
           sinister, threatening]
    2: stemming from evil characteristics or forces; wicked or
       dishonorable; "black deeds"; "a black lie"; "his black heart
       has concocted yet another black deed"; "Darth Vader of the
       dark side"; "a dark purpose"; "dark undercurrents of ethnic
       hostility"; "the scheme of some sinister intelligence bent on
       punishing him"-Thomas Hardy [syn: black, dark,
       sinister]
    3: on or starting from the wearer's left; "bar sinister"

Moby Thesaurus II by Grady Ward, 1.0:

163 Moby Thesaurus words for "sinister":
   adverse, amoral, antagonistic, apocalyptic, backhand, backhanded,
   bad, baleful, baneful, base, black, bodeful, boding, circuitous,
   conflicting, conscienceless, contrary, corrupt, corrupted, counter,
   counteractive, counterclockwise, criminal, crooked, dark,
   deflectional, detrimental, deviant, deviative, devious, diabolical,
   difficult, digressive, dire, disastrous, dishonest, dishonorable,
   divagational, divergent, doomful, doubtful, dreary, dubious,
   evasive, evil, evil-starred, excursive, fateful, felonious, fishy,
   foreboding, fraudulent, furtive, gloomy, hard, harmful, hostile,
   ill, ill-boding, ill-fated, ill-got, ill-gotten, ill-omened,
   ill-starred, immoral, improper, in opposition, inaccurate,
   inauspicious, indirect, inexpedient, inferior, inimical, insidious,
   invalid, larboard, left, left-hand, left-handed, left-wing,
   left-wingish, levorotatory, liberal, lowering, malefic, maleficent,
   malevolent, malicious, malign, malignant, menacing, minacious,
   minatory, miserable, near, nefarious, nigh, not easy, not kosher,
   oblique, of evil portent, ominous, opposed, opposing, opposite,
   peccant, pernicious, port, portending, portentous, questionable,
   radical, rigorous, rotten, shady, shameless, shifty, side,
   sidelong, sinful, sinistral, sinistrocerebral, sinistrocular,
   sinistrogyrate, sinistrorse, slippery, sneaky, somber, stressful,
   suspicious, threatening, treacherous, tricky, troublesome,
   troublous, trying, unconscienced, unconscientious, unconscionable,
   underhand, underhanded, unethical, unfavorable, unfortunate,
   unhealthy, unkind, unlucky, unpleasant, unprincipled, unpromising,
   unpropitious, unsavory, unscrupulous, unskillful,
   unstraightforward, untoward, vicious, villainous, wicked,
   without remorse, without shame, wretched, wrong