lu·​na·​cy | \ ˈlü-nə-sē \
plural lunacies

Definition of lunacy

1 dated

b : intermittent insanity once believed to be related to phases of the moon
2 : wild foolishness : extravagant folly
3 : a foolish act

Keep scrolling for more

Examples of lunacy in a Sentence

Quitting her job was lunacy. His idea was considered total lunacy.

Recent Examples on the Web

These gamers sensed that running the ball with your quarterback under center, to obey traditions that dated back to leather helmets and a time when the forward pass wasn’t even legal, was nothing short of lunacy. Andrew Beaton, WSJ, "The NBA and NFL Have Become Videogames," 6 Nov. 2018 The Cup has driven some men to lunacy, others to bankruptcy. Luke O'brien, Town & Country, "Hot Pursuit," 1 Sep. 2013 On top of this, the reaction of young Elliott to the news that football's coming home would be the exact feeling of lunacy that would greet us all if England do win the World Cup this weekend., "It's Coming Home: 6 of the Best England World Cup Memes of the Summer So Far," 11 July 2018 Despite being an Arizona senator for the past 32 years, he’s done nothing to address the open-border lunacy and invasion that’s increased crime and strained the resources of his own state. Ed Stockly,, "Calendar Letters: Jane Pauley, John McCain, Roseanne Barr and more," 31 May 2018 Three centuries ago, notes the narrator— Willem Dafoe reading a text by the director and Robert Macfarlane —climbing a mountain would have been considered an act of lunacy. Joe Morgenstern, WSJ, "‘Mountain’ Review: Such Great Heights," 24 May 2018 The film’s thesis is that, until three centuries ago, it was deemed an act of lunacy to climb such a mountain. Peter Rainer, The Christian Science Monitor, "Documentary ‘Mountain’ has glorious panoramas," 1 June 2018 There are more than just a few Sapphic suggestions here, along with the religious lunacy, sadism, hallucinations, magic realism and a visual vocabulary rooted in horror. John Anderson, WSJ, "‘Picnic at Hanging Rock’ Review: No Walk in the Park," 24 May 2018 The influences of soul, rock, English folk, jazz, jug band and a heavy dose of industrial English LSD lunacy! Ron Hart, Billboard, "Small Faces' 'Ogdens' Nut Gone Flake' Turns 50: Phil Collen, Peter Frampton, Linda Thompson & More Pay Tribute," 24 May 2018

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

See More

First Known Use of lunacy

1541, in the meaning defined at sense 1a

History and Etymology for lunacy


Keep scrolling for more

Learn More about lunacy

Dictionary Entries near lunacy





luna moth

Luna Park


Statistics for lunacy

Last Updated

25 Nov 2018

Look-up Popularity

Time Traveler for lunacy

The first known use of lunacy was in 1541

See more words from the same year

Keep scrolling for more

More Definitions for lunacy


lu·​na·​cy | \ ˈlü-nə-sē \
plural lunacies

Medical Definition of lunacy

: mental illness also : intermittent mental illness once believed to be related to phases of the moon


lu·​na·​cy | \ ˈlü-nə-sē \

Legal Definition of lunacy

Keep scrolling for more

More from Merriam-Webster on lunacy

Thesaurus: All synonyms and antonyms for lunacy

Spanish Central: Translation of lunacy

Nglish: Translation of lunacy for Spanish Speakers

Britannica English: Translation of lunacy for Arabic Speakers

Comments on lunacy

What made you want to look up lunacy? Please tell us where you read or heard it (including the quote, if possible).


to gather or build up little by little

Get Word of the Day daily email!

Test Your Vocabulary

Homophone Quiz

  • three-bears-two-of-them-look-like-theyre-whispering-to-a-third-bear-who-looks-chuffed-to-be-the-center-of-attention
  • In order to judge how people felt, the senator's office hired a firm to take a ______.
True or False

Test your knowledge - and maybe learn something along the way.

Syn City

Test Your Knowledge - and learn some interesting things along the way.


Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!