The acronym stands for "best friend moment." When two or more friends have done or said the same thing simultaneously. Also occurs in past/future forms.
EX.
Person 1: are you guys going go the party?
Person 2 & 3: yes!
Person 1: you just had a BFM!
EX. 2
Person 1 & 2: (both drop their book at the same time)
Person 1: BFM!
EX. 3 Person 1: should we leave soon?
Person 2 & 3: yes!
Big Furry Monster, I.E., the biggest creature Magic the Gathering has to offer. It takes two halves of the card to summon and 15 black mana, and it's a 99/99 and can only be blocked by three or more creatures. It's the biggest, baddest, nastiest, scariest creature you'll ever see.
"It was big. Really, really big. No, bigger than that. Even bigger. Keep going. More. No, more. Look, we're talking Krakens and Dreadnoughts for jewelry, it was BIG!"
-Arna Kennerild, Skyknight