1. the state of being seriously mentally ill
Usage: legal and medical contexts
2. extremely foolish or unreasonable behavior or thinking
Usage: informal

Examples

• The defendant pleaded not guilty by reason of insanity.
• His insanity was evident in his erratic behavior.
• It would be insanity to drive in this storm.
• The insanity of the plan became clear when it failed.
• She questioned the insanity of working sixteen-hour days.
• The court had to determine his mental state and possible insanity.
• Pure insanity drove him to quit his job without notice.