lunacy

Etymology

It is from English lunatic + -acy.^[2][3][4][5] Etymonline calls it irregularly formed, and Barnhart, Etymonline and Klein take it as a shortening of the adjective with the suffix -cy.^[6][7][8] The adjective is from Old French lunatique (“insane”) and Latin lūnāticus (“moon-struck, epileptic”), from lūna (“moon”),^[7][6][5] because attacks of madness were thought to follow the phases of the moon.^[7][5] Old English had words of its own for this, monaðseocnes (“month-sickness”) for the illness and monseoc (“moon-sick”) for the sufferer.^[6]

Earliest recorded meaning

Serious mental illness, at first of a kind with periods of remission and relapse that were put down to the phases of the moon: 1541, in an Act of Parliament of Henry VIII.^[2][7][9] Etymonline dates the word to the 1540s.^[6]

Semantic evolution

The figurative sense, extreme or reckless folly, is recorded from 1561 in the OED, in a translation of Melanchthon;^[2] Barnhart dates it to 1588 and Etymonline to the 1580s.^[7][6] In law lunacy came to mean any unsoundness of mind that made a person unable to manage their affairs or to enter into civil transactions,^[6][3] as in the historical commission of lunacy, a court's inquiry into a person's sanity, and Master in Lunacy, the officer who investigated it.^[3] From 1600 to 1723 farriers also used the word for a disorder of the temper, behaviour or eyesight of horses and mules.^[2][10] Johnson (1755) gave both "a kind of madness influenced by the moon" and "madness in general", quoting Shakespeare's As You Like It,^[11] and Webster (1828) had the same two senses.^[12] Collins now marks the medical sense as former use.^[13]

Timeline

• 1541 – serious mental illness, at first of an intermittent kind ascribed to the moon.^[2][7][9]
• 1561 – figuratively, extreme folly, or a foolish act.^[2] Barnhart dates this sense to 1588 and Etymonline to the 1580s.^[7][6]
• 1600 – in farriery, a disorder of horses and mules (obsolete by 1723).^[2][10]

Meanings

Noun

1. Formerly, severe mental illness or insanity; originally insanity with lucid intervals, thought to be influenced by the changes of the moon.^[13][9][3]
2. Formerly in law, unsoundness of mind that makes a person legally incapable or not responsible.^[9][3]
3. Extreme folly or foolishness, or a foolish act: Selling the house now would be sheer lunacy.^[13][2][3]

Related words

• lunatic
• lunar

References

1. ^ a b Wiktionary, the free dictionary, https://en.wiktionary.org
2. ^ a b c d e f g h Oxford English Dictionary Online (accessed March 2026), Oxford: Oxford University Press
3. ^ a b c d e f Shorter Oxford English Dictionary on Historical Principles (2002), 5th ed., ed. William R. Trumble and Angus Stevenson, Oxford: Oxford University Press
4. ^ Stevenson, Angus, ed. (2010), Oxford Dictionary of English, 3rd ed., Oxford: Oxford University Press
5. ^ a b c The American Heritage Dictionary of the English Language (online 2024), 5th ed., Boston: Houghton Mifflin Harcourt
6. ^ a b c d e f g Harper, Douglas. Online Etymology Dictionary, https://www.etymonline.com
7. ^ a b c d e f g Barnhart, Robert K. (1988), The Barnhart Dictionary of Etymology
8. ^ Klein, Ernest. A Comprehensive Etymological Dictionary of the English Language. 2 vols. Amsterdam: Elsevier, 1966–1967.
9. ^ a b c d Merriam-Webster Unabridged Dictionary (2024), online ed., Springfield, Mass.: Merriam-Webster
10. ^ a b Kay, Christian; Roberts, Jane; Samuels, Michael; Wotherspoon, Irené, eds. (2009), Historical Thesaurus of the Oxford English Dictionary, Oxford: Oxford University Press; online edition (2024)
11. ^ Johnson, Samuel (1755), A Dictionary of the English Language, London; 4th ed. 1773; Johnson's Dictionary Online, ed. Beth Rapp Young et al.
12. ^ Webster, Noah (1828), An American Dictionary of the English Language, New York: S. Converse
13. ^ a b c Collins English Dictionary (2023), Glasgow: HarperCollins