Nicki Minaj and Megan Thee Stallion are at each other's throats right now after trading shots on respective diss tracks -- but Nicki's going low ... swiping at MTS' extremities. It all started on Friday, when Megan made good on her promise to continue to fire back at her critics from the Tory Lanez trial ... and released a new track titled "Hiss" that picked up where the last one, “Cobra,” left off. While going off on haters, she also shaded NM.