From Urban Dictionary (note: If you ever want a laugh, look up their definition of 'mumble rap')
Drill Rap
Drill is a branch of rap music that can be compared to trap music. Both are about life in the streets, but while trap music is about the trap (hence the name) and the drug dealing side of things,
drill music is about the violent side of the streets. Drill is aggressive, dark, violent, and about guns and shootings. It's called drill music because the term "drill" refers to killing, doing a hit, or retaliating. For example, if someone says, "We doin a drill" it means someone bouta get shot up. These aggressive themes are evident in music videos.
Drill rap originated in the South Side of Chicago in the early 2010s, and was made popular by Chief Keef,
Lil Reese, Lil Durk, and others. New York is another drill hub in the US. Brooklyn and
UK drill emerged in the mid 2010s. Many Brooklyn drill rappers use UK drill beats. Some popular Brooklyn rappers are Bobby Shmurda,
Sheff G, Sleepy Hollow, and
22Gz. Pop Smoke, who also rapped over drill beats, is arguably the most popular from this region. UK drill originated in the South London district of Brixton, and their wicked beats are used around the globe.