lunatic

UK: /ˈluːnətɪk/ US: /ˈlunətɪk/ ,(lo̅o̅′nə tik)

WordReference Random House Learner's Dictionary of American English © 2025

lu•na•tic /ˈlunətɪk/   n. [countable]

1. an insane person.
2. a person who is extremely foolish or who takes dangerous, unnecessary chances.

adj. 

3. insane;
   crazy.
4. wildly or dangerously foolish.
5. designated for or used by the insane:
   a lunatic asylum.

WordReference Random House Unabridged Dictionary of American English © 2025

lu•na•tic  (lo̅o̅′nə tik), n. 

1. an insane person.
2. a person whose actions and manner are marked by extreme eccentricity or recklessness.
3. Law a person legally declared to be of unsound mind and who therefore is not held capable or responsible before the law.

adj. 

4. insane; demented;
   crazy.
5. characteristic or suggestive of lunacy;
   wildly or recklessly foolish.
6. designated for or used by the insane:
   a lunatic asylum.
7. gaily or lightheartedly mad, frivolous, eccentric, etc.:
   She has a lunatic charm that is quite engaging.

Also, lu•nat•i•cal  (lo̅o̅ nat′i kəl) (for defs. 4, 5, 7).

• Late Latin lūnāticus moonstruck. See Luna, -atic
• Old French lunatique
• Middle English lunatik 1250–1300

lu•nat′i•cal•ly, adv. 

Collins Concise English Dictionary © HarperCollins Publishers::

lunatic / ˈluːnətɪk/ adjective
also (rarely): lunatical / luːˈnætɪk^əl/

1. mentally ill
2. foolish; eccentric

noun

1. a person with a severe mental illness

Etymology: 13th Century (adj) via Old French from Late Latin lūnāticus crazy, moonstruck, from Latin lūna moon