or Smash-Ass, in this case smash is used to describe an ass that you want to Smash (have sex with). A girl with a nice ass can be referred to as a "Smash Ass" (ass being the primary value of this female). It is only used to describe females.

Smash Ass can also refer to just the ass, if you are so inclined to reserve even a modicum of respect for the lady, you can tell her she has a smash ass (rather than saying she IS a smash ass, reducing her to thing)