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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.

By the third song, Sit Down, Stand Up, they're flexing their musical muscles, with an extended outro of percussive lunacy, aided by US session musician Chris Vatalaro.

From BBC • Nov. 21, 2025

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

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.