insidious.

causing harm gradually or subtly, so that the danger is not readily noticed

Use it for

Words that belong together

• insidious influence
• insidious threat
• insidious onset

On the page

An insidious doubt undermined every decision.

The illness had an insidious onset.

The house's insidious influence became clear only after they left.

Usage notes

Primary curation section: Horror. Emphasizes concealed or creeping harm, not merely evil intention or great severity. Negative connotation. Comparative: more insidious; superlative: most insidious. Related adverb: insidiously.