Best Friends Forever Till Death Do Us Part
Like, being married to your best friend. Being such close friends with someone that only death can separate you two, but only temporary. Shared with your greatest friend, your partner, your companion. Usually shared with friends opposite sex, and that does not mean they are stuck in the 'friend zone'; you are married after all. Can mean much more in the future.
Jimmy: "Hey, you're my best friend, right?"
Jimmy: "Then what are we?"
Julie: "We are bfftddup. Maybe more in the future"
Jimmy: "That's fine by me"