What is the original meaning of insidious?

The original meaning of "insidious" comes from the Latin word "insidēre," which means "to lie in wait," "to ambush," or "to sit down." This sense of lurking or waiting in secret to attack is at the core of the word's original meaning.

Over time, "insidious" evolved to describe something that is "treacherous," "deceitful," or "cunning." It emphasizes the element of stealth and gradual harm, making it difficult to detect the danger until it's too late.

So, the original meaning of "insidious" is closely related to the idea of a hidden threat that works its way into a situation without being noticed. This is why we use the word today to describe things that are slowly and subtly harmful, like a disease that progresses without noticeable symptoms, or a person who manipulates others for their own gain.