This is how Chicago mobsters describe business activities that are not illegal. Gangsters often invest ill-gotten monies into legitimate businesses, which, in turn, generate what they consider to be legitimate profit. Such business are referred to being "on the legit" (despite being an essential part of a larger money-laundering enterprise).
Guy #1: We'll fill the joint with high-line Spanish broads, and keep it on the legit.
Guy #2: Ok, buddy.