insidious

adjective

in·​sid·​i·​ous in-ˈsi-dē-əs 

1

a

: having a gradual and cumulative effect : subtle

the insidious pressures of modern life

b

of a disease : developing so gradually as to be well established before becoming apparent

2

a

: awaiting a chance to entrap : treacherous

b

: harmful but enticing : seductive

insidious drugs

insidiously adverb

insidiousness noun

Did you know?

insidious, sinister, or pernicious?

Few would choose to be associated with people or things that are insidious, sinister, or pernicious; all three of these words have decidedly unpleasant meanings, each with its own particular shade of nastiness.

Insidious comes from a Latin word for “ambush” (insidiae), which is fitting, as this word often carries the meanings “deceitful,” “stealthy,” or “harmful in an imperceptible fashion.” The first two meanings may be applied to people or things (“an insidious enemy,” “an insidious plot”), while the last is usually applied to things (“insidious problems,” “insidious sexism”), in particular to the gradual progress of a disease (“an insidious malignancy”).

Sinister comes from a Latin word meaning “on the left side, unlucky, inauspicious.” Although it is commonly used today in the sense “evil” (“a sinister cult leader”; “a sinister plot”), it may also suggest an ominous foreshadowing of some unfavorable turn of events (“a sinister omen”).

Pernicious has largely stayed true to its etymological root, the Latin noun pernicies “ruin, destruction.” Its original meaning in English, “highly injurious or destructive,” usually applies to things (“pernicious apathy,” “pernicious effects”) and medical conditions (“pernicious fever,” “pernicious anemia”). When applied to people, pernicious means “wicked.”

Examples of insidious in a Sentence

But the litigation is also prompting a subtle and insidious change in the way that medicine is practiced, which affects anyone who consults a health professional, even if they would not dream of setting foot in a lawyer's office. It is known as "defensive medicine." —Geoff Watts, New Scientist, 23–29 Oct. 2004

Spin is sometimes dismissed as a simple euphemism for lying. But it's actually something more insidious: indifference to the truth. —Michael Kinsley, Time, 25 Dec. 2000–1 Jan. 2001

As these boats aged and bedding compounds deteriorated, the water torture began, which led to rot, corrosion, and other insidious problems. —Ralph Naranjo, Cruising World, April 1999

Most people with this insidious disease have no idea that they are infected.

Recent Examples on the Web

But psychological research presents a deeper, more insidious explanation — one that doesn’t just apply to politicians and public figures, but to clinicians, as well. —Robert Pearl, Forbes.com, 26 May 2025

Beyond the physical abuse, Cassie endured a steady drumbeat of insidious threats that sent an unmistakable message: the violence could escalate at any moment. —Lauren Hersh, New York Daily News, 21 May 2025

Here, though, the spectacle of Laura’s darkness is not presented as a slowly unfolding insidious revelation. —Owen Gleiberman, Variety, 16 May 2025

And that’s indeed already happening, probably in insidious ways that aren’t publicly disclosed. —Kieran Press-Reynolds, Pitchfork, 14 May 2025

Word History

Etymology

Latin insidiosus, from insidiae ambush, from insidēre to sit in, sit on, from in- + sedēre to sit — more at sit

First Known Use

1545, in the meaning defined at sense 2a

Rhymes for insidious

• fastidious
• invidious
• hideous

Kids Definition

insidious

adjective

in·​sid·​i·​ous in-ˈsid-ē-əs 

1

a

: awaiting a chance to trap : treacherous

an insidious foe

b

: harmful but attractive

an insidious habit

2

: having an effect that develops gradually

an insidious disease

insidiously adverb

insidiousness noun

Medical Definition

insidious

adjective

in·​sid·​i·​ous in-ˈsid-ē-əs 

: developing so gradually as to be well established before becoming apparent

an insidious disease

insidiously adverb