This could by hyperthyroidism and your pet should go to the vet. Other possibilities could be parasites. Your cat needs a well-check and a CBC (Complete Blood Count) done, which is a standard thing to do. Usually this will mean your cat needs a fairly affordable hormone, a cheap piller to administer it every day, and to no longer be on dry food. Instead, your cat will need a high quality, grain free, high animal protein wet food diet. You have a classic case of hyperthyroidism here.
If your cat is constantly drinking and seeking out water, making big pees, etc, and losing weight, your cat may have kidney failure. That is another possible issue.
Weight loss can also mean cancer.
No matter what, you need to get to the root of the problem and determine what you think can and should be done so your cat doesn't suffer or can have a meaningful recovery.