Short for best friend, but Gen Z uses it loosely as a friendly address for anyone, including strangers on TikTok or in the comments. Your kid might call you bestie one minute and a barista the next; it does not mean a deep friendship. It can be sincere or mildly sarcastic depending on tone. There is nothing concerning about this word on its own. Treat it like sweetie or buddy with a TikTok accent.