When you're so down bad that you need an extension of the phrase to define just exactly how down bad you are. This version of down bad doesn't always have to be applied sexually. It could mean you're down bad for a delightful date, or you're just doing shitty in life overall.
In other words, you're either doomed or desperate as hell.
Having an unusually large, yet attractive behind that has acquired some very appealing looks over the course of ones life, making it seem as if it were bodacious.