Definition of insidious adjective from the Oxford Advanced Learner's Dictionary

 

insidious

 adjective
adjective
BrE BrE//ɪnˈsɪdiəs//
 
; NAmE 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 Word Origin mid 16th cent.: from Latin insidiosus ‘cunning’, from insidiae ‘an ambush or trick’, from insidere ‘lie in wait for’, from in- ‘on’ + sedere ‘sit’.
See the Oxford Advanced American Dictionary entry: insidious