Meaning of “insidious” - Learner’s Dictionary

insidious

adjective uk us /ɪnˈsɪdiəs/

having harmful effects that happen gradually so you do not notice them for a long time:

the insidious effects of pollution
insidiously adverb
insidiousness noun [ U ]

(Definition of “insidious” from the Cambridge Learner’s Dictionary © Cambridge University Press)