in ‧ sid ‧ i ‧ ous /ɪnˈsɪdiəs/ BrE AmE adjective formal
[ Date: 1500-1600 ; Language: Latin ; Origin: insidiosus , from insidiae 'attack from a hiding-place' , from insidere 'to sit on, lie in wait' , from sedere 'to sit' ]
an 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]