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English definition of “insidious”

insidious

adjective     /ɪnˈsɪd.i.əs/
(of something unpleasant or dangerous ) gradually and secretly causing harm : High-blood pressure is an insidious condition which has few symptoms .Dangerous and harmful insidiously     /-li/ adverb Serious and unpleasantNot attractive to look atInformal words for bad insidiousness     /-nəs/ noun [U]
(Definition of insidious adjective from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)
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