The term "mysterious girl" is a figure of speech used to describe someone who has an air of mystery or intrigue about them. This person may be difficult to read, keeping their thoughts and emotions hidden from others, making it hard for people to understand them fully. They could also have a mysterious past or background that adds to the allure around them. In essence, being called "mysterious girl" is often used as a compliment, suggesting that someone has an enigmatic quality about them which can be both fascinating and attractive.