From Longman Dictionary of Contemporary Englishinsidiousin‧sid‧i‧ous /ɪnˈsɪdiəs/ adjective formal BADan insidious change or problem spreads gradually without being noticed, and causes serious harm an insidious trend towards censorship of the press —insidiously adverb —insidiousness noun [uncountable]
Examples from the Corpus
insidious• A more insidious form of water pollution is chemicals used on farms that get into the water supply.Origin insidious (1500-1600) Latin insidiosus, from insidiae “attack from a hiding-place”, from insidere “to sit on, lie in wait”, from sedere “to sit” 