Something like "You have become stagnant with your work."
Stagnant meaning: characterized by lack of development, advancement, or progressive movement.
Initially, I would not have understood what you meant by "Live off old fat". I would have needed some help with the surrounding words.
However, now that I am thinking about it, I do remember my elders saying "Quit living off of your OWN fat and get a job". I believe they were implying that eventually you are going to run out of resources and find yourself wanting. Hope this help