The acronym STFU stands for "shut the f*** up." While it can be perceived as rude or aggressive, its meaning depends heavily on the context and the relationship between the sender and receiver. If it's sent by a girl, it could range from playful teasing among close friends to genuine anger or annoyance. Consider the overall tone of the conversation and your relationship with the sender to understand the intended meaning. It's always best to clarify if you're unsure.