BFF is an abbreviation of "Best Friend(s) Forever". For best spoken results, spell it out loud instead of trying to pronounce it as a single word. In a sentence, you could use it to indicate the nature of a relationship ("She and I are definitely BFF"; "Tomorrow night my BFF Sarah and I are going out for dollar shots"); I've also seen it used as a closing salutation for e-mail ("I'll see you Friday. BFF!"). The capslock means it's serious.

When I was in junior high school, my best friend of the time gave me part of a pendant in pieces that, when assembled, read "BFF". (My section had an "F" enclosed in one-half of a heart shape.) I accepted the gift gratefully, but had no idea what the abbreviation stood for and didn't want to ask, lest I offend the giver. Not long afterward I misplaced the pendant, moved to a different city, and amassed a new collection of BFF; but I didn't know what it meant until last year.

To this day I'm racked with doubt about my understanding of its meaning, and avoid using it at all costs.

for BrevityQuest07 and also hapax who is totally my BFF