Green’s Dictionary of Slang

1. (US black) a pimp.

1931		‘Low Rider’s Blues’ [song title].
1981		J. Ellroy Brown’s Requiem 68: The lobby was crowded with elderly pensioners [...], black prostitutes, and low riders drinking beer.

2. (US prison) an inmate who intimidates other prisoners into paying protection money.

1971		F. Hilaire Thanatos 167: You’ve probably heard them called ‘low-riders.’ [...] three of them drove on me yesterday and said I had to be their kid.
1972		in Wentworth & Flexner DAS (1975) 720/1: There are what the blacks call low riders, these people who run around pushing their weight around, threatening the weak...