American English

Definition of insidious adjective from the Oxford Advanced American Dictionary

 

insidious

 adjective
adjective
NAmE//ɪnˈsɪdiəs//
 
(formal) (disapproving)
 
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spreading gradually or without being noticed, but causing serious harm the insidious effects of polluted water supplies
insidiously
 
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adverb This kind of advertising acts insidiously on young minds.
See the Oxford Advanced Learner's Dictionary entry: insidious