Corpulence is a formal, rather old noun for fatness — the state of having a large, heavy body with a lot of fat. A doctor in an old report might write of a patient’s corpulence. In ordinary talk today, people just say someone is overweight, obese, or fat. Corpulence can sound polite, medical, or a little like a word from an old novel, as if you are naming the condition without pointing a finger. It is not a kind everyday word, and it is not a children’s word. You would sound strange saying it at the dinner table.