When you meet a friend of a friend that you have heard so much about them that you feel that you already know them as a person. Feeling that you are already best friends even if you have only met them twice.
Ex. Your mutual friends sharing inside jokes and then you all share the inside joke, even if the third party was not present when the said joke was made.
Best Friend By Association (BFBA) Scenerio:
Friend: Hey this is my friend Bob
Friend of a Friend: Oh your Bob? I've heard so much about you!!!! (Super excited, and might even share an inside joke that she heard from your mutual friend)
Bob: It's great to finally meet you too, We are like Best Friends by Association (BFBA)