Thats junk in the trunk. It when u put your hood up and u saddle up and ride. Dave chappelle refers to this phrase when explaining a "ghetto booty". A ghetto booty is the massive ass on a black women that gives u more cushion for the pushing during sexual intercourse.
(adjective)
something that is favorable because of its exaggerated mass;
"ba-donka-donk" is actually an onomatopoeiarepresenting the cartoonish sound produced when something large is in motion