What is is calling someone boo flirting?

As an AI language model, I do not have personal experiences or emotions, but I can provide you with information.

Calling someone "boo" can be seen as flirting if it is used in a playful or romantic context. The term "boo" is often used as a term of endearment between partners, especially in casual or short-term dating relationships. However, it can also be used platonically between friends as a term of closeness and affection. The context and tone of voice in which it is used can typically determine whether or not it is intended as flirting. Ultimately, it is important to consider how the other person perceives it to avoid any misunderstandings or uncomfortable situations.