1. A child that is conceived from semen that drips from the anus, over the taint, and into the vagina after anal sex. The resulting mix of jizz and fecal material resembles marble cake mix.
2. A term used to insult a person who may or may not fit definition one.
1. Dude: I blew a huge load up that chick's ass. I hope she doesn't end up pregnant with a marble cake baby.
2. Tom: That guy is a complete douchebag!
Brett: I'll bet he is a marble cake baby.