lunacy

UK: /ˈluːnəsi/ US: /ˈlunəsi/ ,(lo̅o̅′nə sē)

Inflections of 'lunacy' (n): npl: lunacies

WordReference Random House Learner's Dictionary of American English © 2026

lu•na•cy /ˈlunəsi/   n., pl. -cies. 

1. [uncountable] insanity;
   mental disorder.
2. extreme foolishness or an instance of it:
   [uncountable] The decision to resign was sheer lunacy.
   [countable] sick of their lunacies.

WordReference Random House Unabridged Dictionary of American English © 2026

lu•na•cy  (lo̅o̅′nə sē), n., pl. -cies. 

1. insanity;
   mental disorder.
2. intermittent insanity, formerly believed to be related to phases of the moon.
3. extreme foolishness or an instance of it:
   Her decision to resign was sheer lunacy.
4. Law unsoundness of mind sufficient to incapacitate one for civil transactions.

• lun(atic) + -acy 1535–45

Synonyms

• 1. derangement, dementia; craziness, madness, mania, aberration.
• 3. folly, stupidity.

Antonyms

• 1, 2. rationality, sanity.

Collins Concise English Dictionary © HarperCollins Publishers::

lunacy / ˈluːnəsɪ/ noun ( -cies)

1. (formerly) any severe mental illness
2. foolishness or a foolish act