According to Cadillac, he got his nickname because he's "smooth", he "drives straight", and he's "got power, personal power", and it took him "29 years to figure it out".
Cadillac was in jail for getting caught commiting a large money-related crime that he called "The Big Dirty". Had it succeeded, he would've retired from crime and live a life of luxury.
When Ricky and Julian went to jail, Cadillac ended up befriending Julian. He convinced him not to do a Big Dirty, but to instead steal change, as it is untraceable. Julian took this advice.