dank is typically used to describe awesome weed. in this context, it achieves a new superlative meaning to relate to the butt.
it can also be split into the term 'dankiest donk' to describe the most amazing ass in the world.
"yo bro you see that dankydonk over der? tryna."
a: "shit bra she gotta dankydonk. how you ever hit that? she too blind to see you gotta pooface?"
b: "nah b she don't got no dankydonk. pshhh she got the dankiest donk! and don worry she seen mah pooface so well that she know it the shit."