insidious
Americanadjective
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stealthy, subtle, cunning, or treacherous
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working in a subtle or apparently innocuous way, but nevertheless deadly
an insidious illness
Usage
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.
Other Word Forms
Etymology
Origin of insidious
First recorded in 1535–45; from Latin insidiōsus “deceitful,” from insidi(ae) (plural) “an ambush” (from insidēre “to sit in”) + -ōsus -ous; see also sit 1
Explanation
If something is slowly and secretly causing harm, it's insidious — like the rumors no one seems to listen to until suddenly someone's reputation is ruined. Insidious is related to the Latin noun, īnsidiae meaning "ambush" which comes from the Latin verb, īnsidēre "to lie in wait for." This is very fitting as an insidious rumor or problem is one whose negative effect is not realized until the damage is already done. Similarly, an insidious disease develops internally without symptoms, so that you don't realize right away that you are sick.
Vocabulary lists containing insidious
Grade 9, List 1
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300 Most Difficult "SAT" Words
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Fahrenheit 451
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Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
Her cancer is more insidious and the outcome is not as good, but she’ll come out on the other side once again.
From Los Angeles Times • Jul. 7, 2026
Instead, it has laid the groundwork for an even more insidious form of control.
From Salon • Jun. 21, 2026
One might assume that if these bias response teams were so insidious and widespread, there would be other cases in which students had been unfairly targeted by public universities, providing a clearer instance for standing.
From Slate • Jun. 2, 2026
Anticipated regret—FOMO—is insidious, but it isn’t as strong as the fear of losing your livelihood.
From The Wall Street Journal • Jun. 2, 2026
‘And how come? Why has this miracle of ignorance endured? Because of one thing. A conspiracy. A vast and insidious conspiracy. Obscurantism.’
From "The Heart is a Lonely Hunter" by Carson McCullers
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
