Bobbi Kristina -- New Docs Claim BF Nick Gordon Drugged and Drowned Her

Bobbi Kristina died after her boyfriend gave her a toxic cocktail, and when she fell into unconsciousness he put her face down in the bathtub ... so claims the lawyer who represents Bobbi Kristina's estate.

The attorney filed a $40 million lawsuit against Nick Gordon in June, but court documents just filed in the case outline shocking allegations that, if true, would clearly amount to homicide.

The lawyer says at 6 AM on January 31st -- hours before Bobbi Kristina was found face down in the tub -- Nick Gordon came home from an all-night cocaine and alcohol bender. The suit claims he reviewed camera footage and became irate.

The suit says Gordon began screaming at Bobbi Kristina, accusing her of cheating and calling her a "whore and bitch."

Their argument then went from room to room, and at some point the suit claims Gordon gave Bobbi Kristina a "toxic cocktail" that rendered her unconscious.

The lawyer then says Gordon dragged Bobbi Kristina to the bathroom where he placed her face down in a tub filled with cold water.

According to the docs, Gordon got in bed, put his head on a female guest's ankle and said, "Now I want a pretty little white girl like you."

Fifteen minutes later the suit says another person (presumably Max Lomas) discovered Bobbi Kristina in the tub.

The suit also says when Bobbi Kristina was pulled from the tub, one of her teeth was hanging loosely from her mouth, indicating she had been injured.

And this is odd ... the suit says authorities found a dust pan at the bottom of the tub.