It absolutely exploded in the 80s. Perhaps telling enough, NWA was about the only ones pushing the limits with violence.... I guess it worked....
Most of the stuff though was self braggadocious (sexuality generally) or just flat out comedic in nature. Not sure when it became 'cool' to rap about shooting cops, killing your street competition, or raping women but hey.... "progress".
we know where it started! Inner cities, the gangs self loathed (gang rap).