The antonyms for the word "un-fitness" are fit, healthy, strong, and able. Being fit is the opposite of being unfit, which means physically and mentally capable of performing various tasks without difficulty. A healthy person is free from illness or disease and maintains a good physical and mental condition. Strong means having the necessary physical strength to perform different activities or tasks with ease. Able denotes having the capability or potential to carry out something successfully. Therefore, being fit, healthy, strong, and able are the antonyms of un-fitness, which are necessary for leading a happy and fulfilled life.