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English definition of “insidious”

insidious

adjective  /ɪnˈsɪd·i·əs/ us  
(of something unpleasant or dangerous) gradually and secretly causing harm: Cancer is an insidious disease.
insidiously
adverb  /ɪnˈsɪd·i·əs·li/ us  
His negative attitude slowly and insidiously spoiled the atmosphere around the office.
(Definition of insidious from the Cambridge Academic Content Dictionary © Cambridge University Press)
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