BFD stands for Big Fucking Deal. For example, let's use this conversation from a typical American family:
"Timmy's got a D minus in Math, John. Aren't you going to help him? Turn off that goddman tube!"
"Mom, I'm fine. Who needs to know that shit anyways?"
"Jeff, go make your son do his work!"
"Timmy's got a D- in Math. BFD. When I was his age, I was stealing computers from little old ladies on a regula' basis. Come on!"
Or:
This node just got downvoted. BFD.
(See also big deal, who cares?).