lu·na·cy

/ˈluːnəsi/ noun [noncount]

1 a : extreme foolishness

Quitting her job was an act of sheer/pure lunacy. [=madness]

1 b : something that is very foolish

Quitting her job was lunacy.

It would be lunacy [=madness] to try driving home in this storm.

His idea was considered total/sheer/complete lunacy.

2 old-fashioned : extreme mental illness : ↑insanity