Definition
Having an abundance of money; rich, very prosperous.
How People Actually Use It
doing well, flush, usually with cash money, possibly with other resources. e.g. When it comes to oil, Saudi Arabia is rollin' in it. When it comes to good headies, Humboldt County is rollin' in it. The etymology predates the now common ecstasy usage/reference, wherein X'ing or being high on ecstasy is referred to as rolling. (rollin')
"His operations were all making money, he was rollin' in it. When it comes to oil Saudi Arabia is rollin' in it. When it comes to good headies, Humboldt County is rollin' in it."
Source: Urban Dictionary