Insidious – Developing gradually but with harmful effects; stealthily treacherous or deceitful

(Pronounced in-sid-ee-uhs)

In slid the snake around all of us (insidious). It got more dangerous gradually, without being noticed until it was too late.

Examples of insidious in a sentence

He was an insidious and spiteful man, and should not be trusted.

While she seemed genuine and helpful, she had a much more insidious motive.

Their actions had much more insidious effects than they could've predicted.