Definition of insidiously adverb from the Oxford Advanced Learner's Dictionary

 

insidiously

 adverb
adverb
BrE BrE//ɪnˈsɪdiəsli//
 
; NAmE NAmE//ɪnˈsɪdiəsli//
 
(formal, disapproving)
 
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by spreading gradually or without being noticed, but causing serious harm This kind of advertising acts insidiously on young minds.