in·sid·i·ous /ɪnˈsɪdiəs/ adjectiveadj formal BADan insidious change or problem spreads gradually without being noticed, and causes serious harm 〔变化或问题〕暗中为害的,不知不觉间加剧的 an insidious trend towards censorship of the press 实行新闻审查的潜在趋势
—insidiously adverbadv
—insidiousness noun [uncountableU]