insanity

noun

in·​san·​i·​ty in-ˈsa-nə-tē 

plural insanities

1

dated : a severely disordered state of the mind usually occurring as a specific disorder

2

law : unsoundness of mind or lack of the ability to understand that prevents someone from having the mental capacity required by law to enter into a particular relationship, status, or transaction or that releases someone from criminal or civil responsibility

3

a

: extreme folly or unreasonableness

the insanity of violence

His comments were pure insanity.

b

: something utterly foolish or unreasonable

the insanities of modern life

Did you know?

On the Meaning of Insanity

An oft-quoted bon mot (frequently attributed to Albert Einstein, Benjamin Franklin, or a number of other people who probably never said it) is that insanity may be defined as “doing the same thing over and over and expecting different results.” While the job of lexicographers might be easier if they were allowed to use witty sayings instead of precise definitions, such belletristic labors are not within their purview. The straight facts about insanity are these: it has been in use in English since the late 16th century, for the first two hundred years or so carrying only the literal meaning “the condition of being mentally deranged.” In the 19th century it began to take on a looser sense, “extreme folly or unreasonableness.” In modern usage, insanity may be found in both senses: you may tell your brother that trying to skateboard while holding onto a car is "insanity" (in which case you mean that it is extremely foolish) or encounter the stricter original meaning in such contexts as insanity plea.

Synonyms

• aberration
• dementia
• derangement
• lunacy
• madness
• mania
• rage [archaic]

Examples of insanity in a Sentence

She was found not guilty by reason of insanity.

His friends thought his decision to quit his job was pure insanity.

Please, no more violence. It's time to stop this insanity.

the insanities of modern life

Recent Examples on the Web

Sending yet another one back to City Hall would be the definition of insanity. —Whitney Tilson, New York Daily News, 5 June 2025

Alissa pleaded not guilty by reason of insanity, but he was found competent to stand trial in 2023 after spending time at a mental hospital. —John Bacon, USA Today, 4 June 2025

In practice, the 24-player races may not feel entire untenable, but there’s definitely another layer of insanity. —Christopher Cruz, Rolling Stone, 3 June 2025

The gunman was sentenced to life in prison for murder after a jury rejected his attempt to avoid prison time by pleading not guilty by reason of insanity. —Colleen Slevin, Los Angeles Times, 2 June 2025

Word History

First Known Use

1590, in the meaning defined at sense 1

Phrases Containing insanity

• insanity plea

Rhymes for insanity

• humanity
• inanity
• profanity
• urbanity
• christianity
• inhumanity
• sanity
• vanity

Kids Definition

insanity

noun

in·​san·​i·​ty in-ˈsan-ət-ē 

plural insanities

1

: the condition of being mentally unsound especially when serious enough to keep one from being convicted of a crime or from performing duties required by law

2

a

: extreme foolishness or unreasonableness

b

: senseless conduct

Medical Definition

insanity

noun

in·​san·​i·​ty in-ˈsan-ət-ē 

plural insanities

1

dated : a severely disordered state of the mind usually occurring as a specific disorder

2

law : unsoundness of mind or lack of the ability to understand that prevents one from having the mental capacity required by law to enter into a particular relationship, status, or transaction or that releases one from criminal or civil responsibility

Legal Definition

insanity

noun

in·​san·​i·​ty

1

: unsoundness of mind or lack of the ability to understand that prevents one from having the mental capacity required by law to enter into a particular relationship, status, or transaction or that releases one from criminal or civil responsibility: as

a

: a disease, defect, or condition of the mind that renders one unable to understand the nature of a criminal act or the fact that it is wrong or to conform one's conduct to the requirements of the law being violated

b

: inability to understand and participate in legal proceedings brought against one : incompetence

c

: inability to understand the nature and purpose of a punishment (as the death penalty) to which one has been sentenced

d

: inability to understand the nature and consequences of one's acts (as making a will) or of events, matters, or proceedings in which one is involved see also commitment, durham rule, irresistible impulse test, m'naghten test, not guilty by reason of insanity, substantial capacity test compare capacity, competency, competent, diminished capacity, sanity

2

: the affirmative defense of having acted while insane