To be insane is to be mentally ill. This is also a slang word for acting wacky or wild.

This word has two closely related meanings. People who are insane are suffering from mental illness, which is very serious. When someone commits a crime, it’s important to know if they were sane or insane. If they’re insane, the penalty is different. Many uses of the word are not so serious. A wild party is called insane, and car salesmen say their prices are insane. A teacher assigning six hours of homework might be told “That’s insane!”

Definitions of insane
  1. adjective

    afflicted with or characteristic of mental derangement

    “was declared
    insane laughter”


    not consistent with or using reason

    not reasonable; not showing good judgment
    amok, amuck, berserk, demoniac, demoniacal, possessed

    frenzied as if possessed by a demon
    around the bend, balmy, barmy, bats, batty, bonkers, buggy, cracked, crackers, daft, dotty, fruity, haywire, kookie, kooky, loco, loony, loopy, nuts, nutty, round the bend, wacky, whacky

    informal or slang terms for mentally irregular
    brainsick, crazy, demented, disturbed, mad, sick, unbalanced, unhinged

    affected with madness or insanity
    certifiable, certified

    fit to be certified as insane (and treated accordingly)
    crackbrained, idiotic

    insanely irresponsible
    crazed, deranged, half-crazed

    driven insane
    fey, touched

    slightly insane

    suffering from a form of schizophrenia characterized by foolish mannerisms and senseless laughter along with delusions and regressive behavior
    lunatic, moonstruck

    insane and believed to be affected by the phases of the moon
    maniac, maniacal

    wildly disordered

    suffering from a disorder characterized by alternating mania and depression

    resembling the mania of manic-depressive illness
    mentally ill, unsound, unstable

    suffering from severe mental illness
    non compos mentis, of unsound mind

    not of sound mind, memory, or understanding; in law, not competent to go to trial

    suffering from paranoia
    psychopathic, psychopathologic, psychopathological

    suffering from an undiagnosed mental disorder

    characteristic of or suffering from psychosis
    raving mad, wild

    talking or behaving irrationally

    suffering from some form of schizophrenia
    screw-loose, screwy

    not behaving normally
    see moresee less



    mentally healthy; free from mental disorder

    consistent with or based on or using reason
    compos mentis, of sound mind

    of sound mind, memory, and understanding; in law, competent to go to trial
    in her right mind, in his right mind, in their right minds

    behaving responsibly

    having a clear mind
    show more antonyms…

  2. adjective

    very foolish

    insane risks behind the wheel”
    harebrained, mad


    devoid of good sense or judgment

Word Family