Messy Marv, a rapper who gained vast acclaim in the 90s, routinely included the word "messy" in his lyrics to affectionately describe individuals with a tendency to behave in an amusing manner that some may classify as unruly, dramatic, or controversial.
In the 70s and 80s, the word "slay" was redefined to describe and encourage people "killing it" with their moves and physical appearance on the dance floor.
The two were combined to create a term entitled "messy slaying" which translates to someone who is so attractive or successful in their endeavors that it sparks drama, incites mass controversy, or draws attention.
Girl, you messy slaying in that outfit, you not goin home alone tonight.