'Grand'(adjective) means big, large, or huge. 'Hustle'(verb or noun) literally means to hurry or work harder. However, used in the urban connotation, hustle
means to grind
or to get something quickly. That something could be money or just anything one wants or needs. So, grand hustle is defined as being on a big mission to get what you want or need.
I'm on a grand hustle to get money for college.