1(a). To signal (the driver of a racing car) to proceed immediately to the pits.
1(b). The act of hanging a black flag from your car window to let other divers know to get the hell out of your way and take their asses back to the pit stop.
2. The act of waving a gun before discharge.
Rick Ross: "Yo let's go tear through town in your Benz and intimidate people. We need to get out here grinding."
DJ Khaled: "Word We'll go Black Flagging today at 4:20; we takin' over! deuces!."