This term is used when some one simply cannot stop shaking their/this ass/asses. And in some circumstances can be said as "Cant Stop Shaking That Ass".
Damn lil mama CSSTA!
I just CSSTA.
Their just a bunch of whores who CSSTA.
Refers to popular ballad sung by bards in Westeros (Game of Thrones setting) which narrates the annihilation of Lord Castamere by Tywin Lannister. More in general, the expression is used as a warning or a threat.
Yesterday my momsaid that I was in for a bad surprise -- She Rains of Castamered me.