lunacy
Americannoun
-
insanity; mental disorder.
- Synonyms:
- aberration, mania, madness, craziness, dementia, derangement
- Antonyms:
- sanity, rationality
-
intermittent insanity, formerly believed to be related to phases of the moon.
- Antonyms:
- sanity, rationality
-
extreme foolishness or an instance of it.
Her decision to resign was sheer lunacy.
-
unsoundness of mind sufficient to incapacitate one for civil transactions: a former legal term.
noun
-
(formerly) any severe mental illness
-
foolishness or a foolish act
Other Word Forms
Noun Inflected Forms
Etymology
Origin of lunacy
Explanation
Lunacy is acting silly or stupid. Your mom might yell, "It's lunacy to go out in the snow wearing those flip flops!" Some people use the noun lunacy when they talk about actual mental instability, although this is considered old fashioned and outdated. It's much more common to describe the lunacy of someone who does something ridiculous or foolish. While adopting a cat from the animal shelter might be a great idea, adopting all thirty cats from the shelter would be lunacy. Lunacy comes from lunatic, originally "affected with periodic insanity, dependent on phases of the moon."
Vocabulary lists containing lunacy
Power Suffix: -acy
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And Then There Were None
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The Odyssey
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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.
Stanley Kubrick’s 1964 satirical masterpiece “Dr. Strangelove” brilliantly parodied the lunacy of the nuclear threat during the Cold War.
From Salon • Mar. 10, 2026
In the case of “Nirvanna the Band,” I assume the answer to the second question at some point becomes “no,” but it’s all done with a very persuasive mix of vérité style and can-do lunacy.
From The Wall Street Journal • Feb. 12, 2026
Beware the three Ls: leverage, liquidity and lunacy.
From MarketWatch • Nov. 7, 2025
The company’s commedia dell’arte-style shenanigans have a natural elasticity but farcical lunacy snaps when stretched too far.
From Los Angeles Times • Sep. 5, 2025
Tibe stared at me for some while as if establishing lunacy.
From "The Left Hand of Darkness" by Ursula K. Le Guin
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.
