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

adj 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
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
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adj very foolish

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

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