Insanity (noun)

1. A severe mental illness, typically one involving delusions or hallucinations.
2. Extremely foolish or dangerous behavior.

Origin:

Middle english: from old french insanite, from latin insania, from insanus "insane".

Examples:

1. He was committed to an asylum for insanity.
2. The lawyer pleaded insanity for his client.
3. The witness testified that the defendant was suffering from temporary insanity at the time of the crime.
4. The company's decision to invest in that project was sheer insanity.
5. The market's reaction to the news was nothing short of insanity.