Lunacy/ˈluːnəsi/ Definition 1 of 3Pro Tip 1/3Common Hyperbole Often used to exaggerate a point or show disapproval of an idea or action. Selling your house to buy lottery tickets is pure lunacy! Pro Tip 2/3Casual Expression Used informally to express disagreement or disbelief in casual conversation. You're biking across the country with no training? That's lunacy! Pro Tip 3/3Emotional Impact Conveys strong emotions; can be seen as offensive if directed at a person. When he proposed building a house on the moon, everyone shouted, 'Lunacy!'