Which term is used to describe something that is deceitfully harmful?

The term that describes something that is deceitfully harmful is "insidious." This word is often used in contexts where there is a gradual and subtle influence that can lead to harmful consequences, making it particularly dangerous because the presence or effect of the harm isn't immediately obvious. For instance, an insidious disease might progress without evident symptoms, allowing it to wreak havoc before it’s detected.

The other terms do not capture this specific meaning. "Insipid" refers to something lacking flavor or interest; "inhibitive" pertains to something that restrains or prevents action; and "inert" describes something that is inactive or lacks the ability to move. None of these terms conveys the concept of deceitful harm that "insidious" encapsulates.