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
    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
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    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
    “took insane risks behind the wheel”
    synonyms: harebrained, mad
    devoid of good sense or judgment
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