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".
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.