[sin.is.ter] adj [ME sinistre, fr. L sinistr-, sinister on the left side, unlucky, inauspicious] (15c) 1 archaic: unfavorable, unlucky
2. archaic: fraudulent
3: singularly evil or productive of evil
4. a: of, relating to, or situated to the left or on the left side of something; esp: being or relating to the side of a heraldic shield at the left of the person bearing it b: of ill omen by reason of being on the left
5: presaging ill fortune or trouble
6: accompanied by or leading to disaster -- sin.is.ter.ly adv -- sin.is.ter.ness n syn sinister, baleful, malign mean seriously threatening evil or disaster. sinister suggests a general or vague feeling of fear or apprehension on the part of the observer "a sinister aura haunts the place". baleful imputes perniciousness or destructiveness to something whether working openly or covertly "exerting a corrupt and baleful influence". malign applies to what is inherently evil or harmful "the malign effects of racism".