insidious

adjective

in·​sid·​i·​ous in-ˈsi-dē-əs 

1

a

: causing harm in a way that is gradual or not easily noticed

the insidious nature of fraud

especially : having a gradual and cumulative negative effect

the insidious pressures of modern life

The most insidious form of oppression is that which comes at the hands of your own.

—Janice Gassam Asare

… a measureless land thick with pine forest and kudzu, that insidious vine author James Dickey once called a form of vegetable cancer.

—Dale Maharidge

b

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

[Hepatitis C is] a slow, insidious disease. … On average it takes 20 years for cirrhosis to develop and 30 years for cancer to appear.

—Leonard Seeff, quoted in Better Homes & Gardens

2

a

: awaiting a chance to entrap : treacherous

an insidious enemy

an insidious plot/threat

b

: harmful but enticing : seductive

Cocaine is an astonishingly insidious addictive drug …

—Elizabeth Kaye

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

Not only are relapses unpredictable but scientists have yet to explain the slow and insidious degeneration that often occurs even without new lesions. —Lucinda Rosenfeld, New Yorker, 25 Oct. 2025

Stefani Reynolds—Bloomberg/Getty Images The second, more insidious wave is a strategy of institutional capture. —Andrew Weinstein, Time, 24 Oct. 2025

Tyrer had the insidious condition caused by repetitive head trauma and known to cause aggression, mood swings, depression and paranoia. —Kansas City Star, 17 Oct. 2025

Complicating matters is the insidious Host character (voiced by Len Cunningham’s father, Liam Cunningham), who manipulates the players’ reality. —Jennifer Maas, Variety, 2 Oct. 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