Answer: A saturated solution.
Step-by-step explanation:
Well, you've definitely made coffee that is too sweet!
But, chemically speaking, a saturated solution is one in which the solution contains the maximum amount of solute that can be dissolved. Any further addition of solute will not be able to dissolve in the solution.
This is exactly what is happening with our cup of coffee. The coffee solution already contains the maximum amount of sugar that can be dissolved, and no amount of stirring will allow the excess sugar to dissolve.