# insidious

/ˌɪnˈsɪˌdiəs/ (US) · /ɪnsˈɪdɪəs/ (UK)

Source: https://englishreference.com/word/insidious
Updated: 2026-08-27

## Senses

### 1. gradually harmful

adjective · CEFR C1 · Academic

developing slowly and secretly in a way that is harmful.

For teachers: proceeding in a gradual, subtle way, but with very harmful effects. Often used in medical or political contexts to describe something that is not immediately obvious.

- The disease was insidious, showing no symptoms until it was already very serious.
- High blood pressure is often called an insidious condition because it can damage the heart for years without the patient feeling any different.

### 2. deceptive or treacherous

adjective · CEFR C1 · Formal · Literary

tricky and dangerous because it looks attractive or harmless but is actually a trap.

For teachers: treacherous or crafty; intending to entrap or beguile by appearing harmless or alluring.

- The advertisement used insidious methods to make people spend money they did not have.
- The propaganda was insidious, slowly changing public opinion by presenting half-truths as established facts.

## Examples

- The disease is insidious because it has no early symptoms.
- High blood pressure is often called an insidious condition because it damages the heart without causing obvious pain.
- The most insidious forms of propaganda are those that blend seamlessly into entertainment, slowly altering public perception without ever triggering a critical response.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.