[per.ni.cious] adj [ME, fr. MF pernicieus, fr. L perniciosus, fr. pernicies destruction, fr. per- + nec-, nex violent death--more at noxious] (15c) 1: highly injurious or destructive: deadly
2. archaic: wicked -- per.ni.cious.ly adv -- per.ni.cious.ness n syn pernicious, baneful, noxious, deleterious, detrimental mean exceedingly harmful. pernicious implies irreparable harm done through evil or insidious corrupting or undermining "the claim that pornography has a pernicious effect on society". baneful implies injury through poisoning or destroying "the baneful notion that discipline destroys creativity". noxious applies to what is both offensive and injurious to the health of a body or mind "noxious chemical fumes". deleterious applies to what has an often unsuspected harmful effect "a diet found to have deleterious effects". detrimental implies obvious harmfulness to something specified "the detrimental effects of excessive drinking".