insidious

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adjective in·sid·i·ous \in-ˈsi-dē-əs\

Definition of insidious

1a : awaiting a chance to entrap : treacherousb : harmful but enticing : seductive insidious drugs
2a : having a gradual and cumulative effect : subtle the insidious pressures of modern lifeb of a disease : developing so gradually as to be well established before becoming apparent

insidiously

adverb

insidiousness

noun

See insidious defined for English-language learners

See insidious defined for kids

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 of insidious from the Web

The virus, whose victims included major global companies from Merck & Co. to PAO Rosneft, bore similarities to last month’s global ransomware attack but was in some ways more insidious, security experts say.

— james marson, WSJ, "Cyberattacks Hit Major Companies Across Globe", 27 June 2017
There is an insidious, racially motived ideological belief, that black men in America need to be contained.

— danielle tcholakian, Longreads, "Celebrating a Second Independence Day: A Juneteenth Reading List", 21 June 2017
And it must be defended not merely from war or political calamity, but from that natural, more insidious phenomenon: forgetting.

— parul seghal, The Atlantic, "Arundhati Roy’s Fascinating Mess", 17 June 2017
One of the most insidious tools in CrashOverride manipulates the settings on electric power control systems.

— ellen nakashima, Washington Post, "Russia has developed a cyber weapon that can disrupt power grids, according to new research", 12 June 2017
There was the insidious practice of redlining by banks, and many city neighborhoods had racial-restrictive covenants.

— gene balk / fyi guy, The Seattle Times, "The rise and dramatic fall of King County’s black homeowners", 11 June 2017
Critics say gains in Big Data and artificial intelligence are more insidious in China than the West because China's political system offers far fewer protections for individual rights.

— clay chandler, Fortune, "Does Chinese innovation have a dark side?", 10 June 2017
Beyond white people’s need to deny the racist history of the word is an insidious desire to be part of everything, whether it is meant for them or not.

— monique judge, The Root, "#NeverForget: Bill Maher Once Questioned a Black Woman’s Blackness Over the N-Word", 7 June 2017
This highlights the insidious nature of radicalization.

— annabelle liang, The Seattle Times, "Joint patrols to target Asia’s Islamic militants", 3 June 2017

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'insidious'. Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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.”