Do you know that one friend who knows you better than anyone, your very closest pal? That's your bestie. The word bestie is an informal shorthand for "best friend." It's been used for decades by people describing their closest confidants, but it began appearing in dictionaries as a slang term sometime after 2010. You might use bestie interchangeably with "BFF" to describe the one person you can trust completely to keep your secrets and save a seat for you on the bus no matter what.

Source: Vocabulary.com