Definition of pernicious adjective from the Oxford Advanced American Dictionary

 

pernicious

 adjective
adjective
NAmE//pərˈnɪʃəs//
 
(formal)
 
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having a very harmful effect on someone or something, especially in a way that is gradual and not easily noticed the pernicious influence of TV violence on children Ageism is equally as offensive and pernicious as sexism.
See the Oxford Advanced Learner's Dictionary entry: pernicious