Synonyms for Sinister:

adj.

all (adjective)
dark, black.
dark (adjective)
unpropitious.
dishonest (adjective)
perfidious.
evil (adjective)
hideous, satanic, deleterious, foul, loathsome, atrocious, offensive, unfortunate, bad, dire, wicked, pernicious, unfavorable, terrible, revolting, repulsive, destructive, nefarious, damnable, dishonest, poisonous, abominable, disgraceful, malign, evil, heinous, calamitous, injurious, foreboding, harmful, unpropitious, mischievous, woeful, detrimental, adverse, unlucky, reprehensible, monstrous, Fiendish, devilish, sinful, repugnant, baneful, repellent, diabolical.
harmful (adjective)
pernicious, mischievous.
hateful (adjective)
malevolent.
hopeless (adjective)
black.
left (adjective)
sinistral.
malevolent (adjective)
malignant, churlish, mean, baneful, spiteful, rancorous, treacherous, wicked, foul, venomous, perfidious, evil, bitter, virulent, hateful, vindictive, catty, antagonistic, baleful, ill-natured, malicious, vicious, hurtful, sullen, hostile, corrosive, Black-hearted, nasty, malevolent.
nasty, menacing (adjective)
injurious, dire, malign, harmful, adverse, malevolent, dishonest, hurtful, baneful, deleterious, malignant, unpropitious, unfortunate, woeful, foreboding, corrupt, perverse, threatening, mischievous, poisonous, evil, baleful, unfavorable, obnoxious, bad, pernicious, unlucky, ominous.
threatening (adjective)
minatory, minacious.
wicked (adjective)
improper, criminal, indecent, damnable, sadistic, perverse, shocking, wicked, sinful, villainous, obnoxious, reprehensible, blameworthy, lawbreaking, debauched, Misdoing, corrupt, base, black, evildoing, immoral, wrongdoing, depraved, scandalous, degenerate, debased, bad, iniquitous, felonious.

n.

warn, malign. sinister (noun)
baleful, menacing, sinistral, wicked, ominous, black, minatory, ugly, alarming, dark, threatening, forbidding, evil, minacious.

Other synonyms:

dishonest. malign. injurious
harmful.
malicious
satanic.
ominous
ominous.
Other relevant words:
menacing, sinistral, forbidding, devilish, foreboding, pernicious, injurious, adverse, alarming, dire, threatening, harmful, ugly, unpropitious, unfortunate, warn, mischievous, unlucky, deleterious, dark, minacious, diabolical, ominous, poisonous, Fiendish, malign, satanic, woeful, minatory, unfavorable, dishonest.

Usage examples for sinister

  1. The neighborhood, likewise, seemed sinister – Lefty Locke Pitcher-Manager by Burt L. Standish
  2. His manner was somewhat sinister and disagreeable. – In Indian Mexico (1908) by Frederick Starr