Maniac (noun)

A person who is excessively enthusiastic or obsessed with something, especially to an irrational degree.

Origin:

From the greek word 'maniakos', meaning 'madman' or 'person possessed by a daimon.'.

Examples:

1. He was a complete maniac when it came to cars and would spend hours working on them.
2. She was a manic collector and had every type of glass figurine known to man.
3. The DJ was a maniac when it came to music and would play for hours without stopping.
4. He was a maniac about cleanliness and would clean everything in sight until it shone.
5. She was a maniac about exercise and would work out for hours every day.