When you grind coffee beans the result is fresh grounds. How finely you grind them determines what grind you produce, such as a coarse or medium grind. In this sense the noun “grinds” can refer only to the size of the coffee granules, not to the granules themselves.

Some people like to put their used grounds into the ground to fertilize their plants.

But fresh or used, that ground-up stuff is not “grinds” but “grounds.”