in·sid·i·ous /ɪnˈsɪdiəs/ adjectiveadj formalBADan insidious change or problem spreads gradually without being noticed, and causes serious harm 〔變化或問題〕暗中爲害的,不知不覺間加劇的an insidious trend towards censorship of the press 實行新聞審查的潛在趨勢
—insidiously adverbadv
—insidiousness noun [uncountableU]