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Synonyms

lunacy

American  
[loo-nuh-see] / ˈlu nə si /

noun

plural

lunacies

  1. insanity; mental disorder.

    Synonyms:
    aberration, mania, madness, craziness, dementia, derangement
    Antonyms:
    sanity, rationality

  2. intermittent insanity, formerly believed to be related to phases of the moon.

    Antonyms:
    sanity, rationality

  3. extreme foolishness or an instance of it.

    Her decision to resign was sheer lunacy.
    Synonyms:
    stupidity, folly

  4. unsoundness of mind sufficient to incapacitate one for civil transactions: a former legal term.

lunacy British  
/ ˈluːnəsɪ /

noun

  1. (formerly) any severe mental illness

  2. foolishness or a foolish act

"Collins English Dictionary — Complete & Unabridged" 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Etymology

Origin of lunacy

First recorded in 1535–45; lun(atic) + -acy

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.

Beware the three Ls: leverage, liquidity and lunacy.

From MarketWatch • Nov. 7, 2025

“Bloodline” reminds me of the breed of darkly comic postwar novels by Robert Coover, William H. Gass, Harry Crews and others, in which regional manias stand in for a broader national lunacy.

From The Wall Street Journal • Oct. 17, 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

Is it hard, though, to freeze all the negativity and all the lunacy that has been directed at you?

From BBC • Jul. 21, 2025

Gally’s eyes raged with lunacy; his clothes were torn and filthy.

From "The Maze Runner" by James Dashner

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.