insidious

[ in-sid-ee-uhs ]

adjective

1. intended to entrap or beguile:

   an insidious plan.

2. stealthily treacherous or deceitful:

   an insidious enemy.

   Synonyms: crafty, subtle, wily, cunning, artful

3. operating or proceeding in an inconspicuous or seemingly harmless way but actually with grave effect:

   an insidious disease.

insidious

/ ɪnˈsɪdɪəs /

adjective

1. stealthy, subtle, cunning, or treacherous
2. working in a subtle or apparently innocuous way, but nevertheless deadly

   an insidious illness

Derived Forms

• inˈsidiousness, noun
• inˈsidiously, adverb

Other Words From

• in·sidi·ous·ly adverb
• in·sidi·ous·ness noun
• unin·sidi·ous adjective
• unin·sidi·ous·ly adverb
• unin·sidi·ous·ness noun

Word History and Origins

Origin of insidious¹

First recorded in 1535–45; from Latin insidiōsus “deceitful,” from insidi(ae) (plural) “an ambush” (from insidēre “to sit in”) + -ōsus -ous; sit ¹

Word History and Origins

Origin of insidious¹

C16: from Latin insidiōsus cunning, from insidiae an ambush, from insidēre to sit in; see insessorial

Example Sentences

Though they’re inaudible to human ears, whose range bottoms out at 20 Hz, the interval creates some fairly insidious side effects.

From Popular-Science

Ginsburg delivered her dissent aloud from the bench, accusing her colleagues of either ignoring or failing to comprehend the “insidious” nature of pay discrimination, and called on Congress to act.

From FiveThirtyEight

The most insidious problem, however, is the varroa mite, appropriately named Varroa destructor.

From Nautilus

Indeed, air conditioning represents one of the most insidious challenges of climate change, and one of the most difficult technological problems to fix.

From MIT Technology Review

Such a situation is particularly insidious because the pay gap she experiences would often go undetected.

From Fortune

Is it anti-Semitism, or are less insidious cultural forces at work?

From The Daily Beast

But its far more insidious role was revealed, whether it was gun policies or voter suppression.

From The Daily Beast

The act of erasure through mis- or under-representation is an insidious one.

From The Daily Beast

Its insidious reach enters into medical offices and chokes off the free-speech rights of the people trying to work there.

From The Daily Beast

But the commentary that McCarthy gets from her so-called supporters is often just as cruel, but somehow more insidious.

From The Daily Beast

Play-writing is a luxury to a journalist, as insidious as golf and much more expensive in time and money.

From Project Gutenberg

Each cachet contained three decigrams of malourea, the insidious drug notorious under its trade name of Veronal.

From Project Gutenberg

The state of affairs which has come about was uncertain in origin, insidious in growth, and has developed over a wide field.

From Project Gutenberg

She believed he sought her, and she must needs fight an insidious liking for him.

From Project Gutenberg

This is a thing I would despise in anybody else; but he is so jolly insidious 240 and ingratiating!

From Project Gutenberg

Related Words

• dangerous
• subtle

More About Insidious

What does insidious mean?

Insidious describes something as being sneaky or being secretly dangerous or harmful.

Insidious also describes something as being corrupting or intending to entrap.

The word insidious is used to describe something as being stealthy, treacherous, or operating in the shadows, as in He came up with an insidious plan to rob the jewelry store. Insidious is always associated with something negative, implying something is bad or is going to cause harm.

Insidious is also often used to describe something as appearing harmless or nonthreatening but subtly causing serious damage, as in The insidious language in the book radicalized many readers without them even knowing it.

Related to this sense, insidious is used in medicine to refer to diseases that slowly get worse and worse without any noticeable signs that they are there at all. Cancer is often described as insidious because it is often found when it is too late to treat it.

Example: The investigators were after an insidious group of hackers who were stealing government secrets.

Where does insidious come from?

The first records of insidious come from around 1535. It comes from the Latin insidiōsus, meaning “deceitful.”

The word insidious is often used in the media and popular culture to describe conspiracies, schemes, corruption, shadowy organizations, and other bad things that are done in secret.

Did you know ... ?

What are some other forms related to insidious?

• insidiously (adjective)
• insidiousness (noun)

What are some synonyms for insidious?

• sneaky
• tricky
• dangerous
• subtle

What are some words that often get used in discussing insidious?

• secret
• stealthy
• hidden
• evil
• villainous
• criminal
• disease

How is insidious used in real life?

Insidious is a word that describes something as being sneaky and up to no good.

A lot of attention has been given to WhatsApp’s privacy changes.

But the most insidious features from these companies are non-obvious but right in front of our faces.

— Tariq Patanam (@TariqPatanam) January 12, 2021

Prisons are full of insidious things but one of the most infuriating is that incarcerated people have to pay money to make a phone call. All the research we have says maintaining family ties reduces the likelihood of coming back & yet the system makes it as difficult as possible.

— Clint Smith (@ClintSmithIII) April 1, 2019

Today is National Human Trafficking Awareness Day. An insidious crime that affects so many children and women from vulnerable populations. Need help call 1-800-373-7888. pic.twitter.com/q6oxcZ7ife

— Chief Ken Berkowitz (@ChiefBerkowitz) January 11, 2021

Try using insidious!

Is insidious used correctly in the following sentence?

She was so cunning and crafty that we didn’t have a clue about her insidious plans until after she had betrayed us.