insidious

in-SIH-dee-us

meaning in plain English

Quietly dangerous, it hurts you slowly without you noticing.

Think of a sneaky snake called Insidious, slithering in quietly and biting you slow— you don't feel it until it's too late.

for example

The insidious mold grew behind the bathroom tiles, making the wood rot before anyone saw it.

Imagine a tiny crack in a pipe that you don't hear at first. Over weeks it becomes an insidious leak that can flood a house if you don't catch it early.