In a manner that is gradually harmful or deceptive.

ऐसा तरीका जो धीरे-धीरे हानिकारक या धोखेबाज़ हो।

English Usage: The disease progressed insidiously, showing few symptoms at first.

Hindi Usage: बीमारी धीरे-धीरे बढ़ी, शुरू में कुछ लक्षण दिखाई नहीं दिए।