BFF means someone’s very best friend — originally “best friends forever.” Young people use it in texts, on social media, in magazines, and in speech, often with a warm or playful feeling. You can call a person your BFF, or write it at the end of a message the way you might write love you. Though actually, adults now use it too, sometimes seriously and sometimes as a joke, because it still sounds a bit teenage. It is friendly and informal, not a word for a school essay or a job letter. The “forever” is part of the flavour: it is not only “a good friend,” it is the one you want to keep.

一輩子的好朋友（年輕人也直接說 BFF）