If the person has got extreme cool teeth when she is smiling, or got a face that automatically says to you "that gotta be a sweet little beaver!" BBF is your best friend for life!
!!! - You don't use this word if your friend is not sweet=)
'Best Behaviour Friend'. A friend or acquaintance in your social circle, whom you for one reason or another feel that you should be at your best behaviour around.
"My BBF came to the party, so I had to restrain myself hugely. If he hadn't been there, I'd have drunk so much more, and ended up dancing naked on the tables as I normally do when I've done tequila slammers"