1. a person who is excessively focused on or obsessed with a single idea, subject, or activity

Examples

• He became a monomaniac about collecting vintage cars.
• The professor was a monomaniac when it came to ancient Roman history.
• She called him a monomaniac for spending every weekend working on his model trains.
• The artist was a monomaniac who painted nothing but seascapes for twenty years.
• His friends worried he was becoming a monomaniac about fitness and exercise.
• The detective’s colleagues thought he was a monomaniac in his pursuit of the case.