Meaning of “insidious” in the English Dictionary

"insidious" in British English

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insidiousadjective

uk /ɪnˈsɪd.i.əs/ us /ɪnˈsɪd.i.əs/
insidiously
adverb uk /ɪnˈsɪd.i.əs.li/ us /ɪnˈsɪd.i.əs.li/

The disease begins insidiously and progresses relentlessly.
insidiousness
noun [ U ] uk /ɪnˈsɪd.i.əs.nəs/ us /ɪnˈsɪd.i.əs.nəs/

We must recognise the insidiousness of these racial stereotypes.

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

"insidious" in American English

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insidiousadjective

us /ɪnˈsɪd·i·əs/

(of something unpleasant or dangerous) gradually and secretly causing harm:

Cancer is an insidious disease.
insidiously
adverb us /ɪnˈsɪd·i·əs·li/

His negative attitude slowly and insidiously spoiled the atmosphere around the office.

(Definition of “insidious” from the Cambridge Academic Content Dictionary © Cambridge University Press)