INSIDIOUS Meaning of INSIDIOUS in English [ ɪn'sɪdɪəs ]■ adjective proceeding in a gradual, subtle way, but with harmful effects.Derivatives~ly adverb~ness noun OriginC16: from L. insidiosus 'cunning'. Concise Oxford English vocab. Сжатый оксфордский словарь английского языка. 2004