NBF is a person who has only just become your close friend — a “new best friend.” Young people and magazines use it, often with a little smile. Sometimes it is warm: you met someone yesterday, and you already feel close. Sometimes it teases, as if the friendship came too fast. It is very informal. You would not put it in a school essay. Write the three capital letters, or say the three words. Though actually, “best friend” is the old, serious phrase; the “new” is the point, and it also hints that the feeling may change.