Most common among women, hair twirling gives off multiple signals such as nervousness, boredom or interest. The age of the female and the situation all play a part into why hair twirling occurs.
A young girl that is twirling her hair may also be prone to nail-biting. These nervous habits are external signs of anxiety. If a woman is idle at work or sitting around and twirling her hair, she could be bored and performing the action without thinking. If a woman does this on a date, she's likely trying to indicate interest to her date and may even be flirting.