lunacy

noun

lu·​na·​cy ˈlü-nə-sē 

plural lunacies

1

dated

a

: insanity sense 1

b

: intermittent insanity once believed to be related to phases of the moon

2

: wild foolishness : extravagant folly

3

: a foolish act

Synonyms of lunacy

• insanity
• idiocy

Examples of lunacy in a Sentence

Quitting her job was lunacy.

His idea was considered total lunacy.

Recent Examples on the Web

But asking a vitriolic group of nearly 20,000 San Antonio Spurs supporters to crank up the volume before shooting a potential game-winning free throw with 6.4 seconds remaining? A unique type of lunacy …. —Joseph Dycus, Mercury News, 15 Nov. 2025

Because it’s invariably attached to the idea of some kind of lunacy. —Zack Sharf, Variety, 3 Nov. 2025

Because it's invariably attached to the idea of some kind of lunacy. —Mekishana Pierre, Entertainment Weekly, 3 Nov. 2025

But the lunacy of that fateful afternoon in 1990 didn’t carry over to the present day, and Coleman Jr. was short by inches. —Quentin Corpuel, Kansas City Star, 31 Oct. 2025

Word History

Etymology

lun(atic) + -acy

First Known Use

1541, in the meaning defined at sense 1a

Kids Definition

lunacy

noun

lu·​na·​cy ˈlü-nə-sē 

plural lunacies

: insanity sense 1

—used chiefly in the past

Medical Definition

lunacy

noun

lu·​na·​cy ˈlü-nə-sē 

plural lunacies

dated

: mental illness

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

Legal Definition

lunacy

noun

lu·​na·​cy ˈlü-nə-sē 

: insanity