Definition
proceeding in a gradual, subtle way with harmful effects
In a sentence
“The disease has an insidious onset.”
Similar words
- sneaky
- stealthy
Related vocabulary
Why this word matters: insidious appears in academic reading at the 8th-grade level and is exactly the type of word that shows up on the CPS HSAT Reading section. Practice it in context with our spaced-repetition system at vocabulary.