Merriam-Webster Collegiate Dictionary - pernicious

 adjective  Etymology: Middle English, from Anglo-French, from Latin perniciosus, from pernicies destruction, from per- + nec-, nex violent death — more at noxious  Date: 15th century  1. highly injurious or destructive ; deadly  2. archaic wicked  • ~ly adverb  • ~ness noun Synonyms:  ~, baneful, noxious, deleterious, detrimental mean exceedingly harmful. ~ implies irreparable harm done through evil or insidious corrupting or undermining the claim that pornography has a ~ 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.