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1.
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!
by BRICKESE February 17, 2013