something or someone (most commonly an ugly girl) which/who is both dirty
(in the bad way) and rank
hence the collaboration of both words into the phrase "dirty rank" and then the shortening of the phrase into the word "dank."
"oh man, celia is such a dank cunt!"
"fuck off celia you dank bitch!"
"that celia is so fucking ugly, she's proper dank!"
"that celia is so dank it makes me want to tear my own eyes out!"