|1.||BMFD: Balls Mutha Fucking Deep|
1) When you go all the way in, all the way up to your balls.
Can also mean
2) in a lot of trouble/peril , in a lot of danger.
She likes when he goes BMFD: Balls mutha fucking deep nside her and she feels it poke her back.
The other day i got caught in a street brawl and was Balls mutha fucking deep right in the middle of it!