/ ɪnˈsɪdiəs; NAmE / adjective
( formal , disapproving ) spreading gradually or without being noticed, but causing serious harm :
the insidious effects of polluted water supplies
► in·sidi·ous·ly adverb
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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.