Webster's English Dictionary

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sinister

sin.is.ter \'sin-*s-t*r, archaic s*-'nis-\ aj [ME sinistre, fr. L sinistr-, 
   sinister on the left side, unlu]cky, inauspicious archaic  1: UNFAVORABLE, 
   UNLUCKY  obs  2: conveying misleading or detrimental opinion or advice  
   archaic  3: dishonestly underhanded : FRAUDULENT  4: singularly evil or 
   productive of evil : BAD, CORRUPTIVE  5a: of, relating to, or situated to 
   the left or on the left side of someth ing {bearing a bar ~} 5b: of ill 
   omen by reason of being on the left  6: presaging ill fortune or trouble : 
   OMINOUS  7: accompanied by or leading to disaster or unfavorable 
   developments ng evil or disaster. SINISTER applies to what threatens by 
   appearance or reputation to be formidably troublesome or dangerous in some 
   undisclosed way; BALEFUL imputes perniciousness or destructiveness to 
   something whether working openly or covertly; MALIGN applies to what is 
   inherently evil or harmful - sin.is.ter.ly av SYN syn SINISTER, BALEFUL, 
   MALIGN mean seriously threateni