Webster's English Dictionary

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pernicious

per.ni.cious \p*r-'nish-*s\ aj [MF pernicieus, fr. L perniciosus, fr. 
   pernicies destruction,] fr. per- + nec-, nex violent death - more at 
   NOXIOUS 1: highly injurious or destructive : DEADLY  archaic  2: 
   WICKEDMDETRIMENTAL mean exceedingly harmful. PERNICIOUS and BANEFUL both 
   imply causing irreparable or deadly injury, PERNICIOUS through evil or 
   insidious corrupting or undermining, BANEFUL through poisoning or 
   destroying; NOXIOUS applies to what is both offensive and injurious to the 
   health of body or mind; DELETERIOUS applies to what has an unsuspected 
   harmful effect when eaten or drunk or inhaled; DETRIMENTAL implies obvious 
   harmfulness to something specified - per.ni.cious.ly av SYN syn PERNICIOUS, 
   BANEFUL, NOXIOUS, DELETERIOUS, (