Ford mustangs driven by hot girls causing you to want to fornicate with them due to the fact that they drive such a badass piece of American muscle.
(Pronouced- Sexxxx-Stang
Nicole: I got a new car, check it out.
Kush: Dudes thats a sweetMustang or should i say... SEXSTANG!!!
"Sexy" to pertain to something of sex or intense pleasure. "Thang" a urban hip term whose origination are from the community of Blacks, refers to a thing thats cool.
Hence "Sexy Thang" is used when such perversions are unwanted. It is used especially when specific details of a thing are to be generalized as to avoid provoking a person to think dirty thoughts.