A term of endearment for the sagging fat at the back of a person's arms (tricep area) that hangs and usually jiggles when one waves or moves their arm rapidly. It is termed from the unmistakable resemblance of the typical low-slung underbelly of a cat.
Oh goddness, my cat bellies are so jiggly today!
Person A: "What are you gunna work out today at the gym?"
Person B: "I think I'm gunna work on my cat bellies"