insidious

insidious
/;n;s;d.i.;s/

Definition: Proceeding in a gradual, subtle way but with harmful effects; treacherous.

Origin: Late Middle English, from Latin insidiosus, from insidiae meaning "to lie in wait".

Example: The insidious spread of misinformation can undermine trust in society.

"Treacherous" embodies the essence of "insidious," as both terms convey a sense of hidden danger or deceit. When something is described as treacherous, it suggests a betrayal or a deceptive nature that may not be immediately apparent. Just as insidious actions can appear harmless while causing significant harm over time, treacherous situations often mask their peril, luring individuals into a false sense of security. In essence, both words highlight the danger that lurks beneath a seemingly benign exterior, emphasizing the need for vigilance against unforeseen threats.

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in·sidi·ous
[;n;s;di;s]

adjective
insidious (adjective)
proceeding in a gradual, subtle way, but with very harmful effects:
"sexual harassment is a serious and insidious problem"
Similar:
stealthy
subtle
surreptitious
sneaking
cunning
crafty
Machiavellian
artful
guileful
sly
wily
tricky
slick
deceitful
deceptive
dishonest
underhand
backhanded
indirect
sneaky
Opposite:
straightforward


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insidious
adjective
in·;sid·;i·;ous in-;si-d;-;s
Synonyms of insidious
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
Examples are automatically compiled from online sources to show current usage. Opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback.
Duckworth also faced backlash from critics who argued that her message was, at best, a series of hollow exhortations to simply try harder and, at worst, an insidious form of victim-blaming that ignored structural inequality.
— Meghan O’Gieblyn, New Yorker, 14 Sep. 2026
If anything, this makes those insidious Fridays insignificantly safer than the state average for the period.
— Mike McRae 08, New Atlas, 8 Sep. 2026
Growing up in Beverly, Frates – the warrior against ALS who succumbed to the insidious disease in 2019 – was a huge fan of the Bruins’ star and had a poster of the defenseman in his room.
— Steve Conroy, Boston Herald, 6 Sep. 2026
But the insidious harm that moves in the other direction — from illness, through a patient, into the person treating her — has no forum at all.
— Jennifer Adaeze Okwerekwu, STAT, 1 Sep. 2026