Other forms: diminishingly
Diminishing means becoming smaller and smaller. While diminishing pain is a good thing, diminishing returns on your investment are not so welcome.
Diminishing comes from the Latin word minuere — "to make small." The "law of diminishing returns" refers to the tendency at a certain point for continued effort in an area to begin producing a smaller and smaller result. You might enjoy eating one Sour Patch candy, but if you eat a whole package of them and feel sick? Then, you have come face-to-face with the law of diminishing returns.