insane

American  

[in-seyn] / ɪnˈseɪn /

adjective

1. (not in technical use as a medical diagnosis) not sane; not of sound mind; mentally deranged.

   Synonyms:

   irrational, foolish, maniacal, crazy, crazed, lunatic, demented

2. of, relating to, or characteristic of a person who is mentally deranged.

   a wild and insane look on her face.

3. Law. considered to be or to have been affected with insanity, and therefore lacking legal responsibility or legal capacity.

   The judge deemed him criminally insane on the basis of his delusions and hallucinations.

4. temporarily unable to think clearly.

   I think I'm going insane from all the noise outside.

5. utterly senseless.

   an insane plan.

6. Slang. excessive or extravagant; extreme.

   This much snow in April is just insane!

7. Slang. astonishingly great; wonderful.

   Without his insane performance, the film would've been a total flop.

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insane British  

/ ɪnˈseɪn /

adjective

1. 1. mentally deranged; crazy; of unsound mind

   2. ( as collective noun; preceded by the )

      the insane

2. characteristic of a person of unsound mind

   an insane stare

3. irresponsible; very foolish; stupid

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Related Words

See mad.

Other Word Forms

• insanely adverb
• insaneness noun
• pseudoinsane adjective

Etymology

Origin of insane

First recorded in 1550–60; from Latin insānus “unsound (of mind), demented, frenzied”; see origin at in- ³, sane

Explanation

To be insane is to have a seriously disordered state of mind. This is also a slang word for acting wacky or wild. This word has two closely related meanings. People who are insane are suffering from a seriously irrational or disordered state of mind. When someone commits a crime, it's important to know if they were sane or insane. If they're insane, the penalty is different. Many uses of the word are not so serious. A wild party is called insane, and car salesmen say their prices are insane. A teacher assigning six hours of homework might be told "That’s insane!"

Example Sentences

Grieving is also time to get angry and justified in your own insane behavior.

From Slate • Apr. 23, 2026

Kahl felt like an insider because she had randomly come across the insane deal while shopping on Google Flights the night before.

From The Wall Street Journal • Apr. 22, 2026

It sounds borderline insane to consider that the only available team in America’s second-largest market might not be worth as much as the team that just sold in America’s 30th-largest market.

From Los Angeles Times • Apr. 17, 2026

“All of us are going through this insane geopolitical time,” Sullivan says.

From Barron's • Apr. 15, 2026

I manage to brush my teeth, put a bandanna on over my insane hair and get dressed in my catering standard: checked pants and a white men’s T-shirt.

From "Ask the Passengers" by A.S. King

Related Words

• crazy
• delirious
• demented
• deranged
• frantic
• frenzied
• kooky
• nutty