Green’s Dictionary of Slang

drag n.2

[note this version of the dozens n. is characterized by its extreme vulgarity and coarseness]

(US black) a ritual game of testing a rival’s emotional strength by insulting his various relatives.

[US](con. 1930s) C.E. Lincoln The Avenue, Clayton City (1996) 17: Dragging was a variation of the dozens [...] The drag Dr. Tait considered the supreme vulgarity was usually heard several times in the evening’s program of obscenities.
[US]R.C. Cruz Straight Outta Compton 17: Flip kept on talking that drag while we watched Willie sell sugar on the edge of the corner.
[US]Mack 10 ‘Based on a True Story’ [lyrics] Do what he say, and you keep your mouth shut / Talkin’ that drag might get ya fucked up.